Peperonata.

Peperonata from "Divina Cucina" blog.
Or Tuscan Bell Pepper-Red Onion Stew.
Notes:
For the pureed tomatoes, you can use a puree of fresh peeled and seeded tomatoes that have been briefly cooked to concentrate their flavor, or canned whole tomatoes that you've blended with their juices. (Canned tomatoes are often the better choice, since they are usually top-quality.)
If you make this with beautiful, ripe summer bell peppers, they should provide enough sweetness to balance the small amount of vinegar.
If your peppers are less sweet and the peperonata tastes a little too tart, add a tiny bit of sugar, a pinch at a time, until the flavor is balanced.

Don't brown the onion, just let it soften to a pale golden colour.
The texture of the peppers and onions should be silky and soft enough to crush between your fingers.
The stew will mellow when left in the fridge for a day or two.

4 large bell peppers, cut into 1 inch strips
4 large red onions, peeled and cut in half and into slices
olive oil
salt
2 Tbs sugar
2 Tbs red or white wine vinegar.
4 Tbs capers

Drizzle a little olive oil in a large skillet.
Add the peppers and onions.
Sprinkle with salt (this will bring the liquid out of the veggies.)
Cover and let cook until the peppers and onions have become soft.
Add the sugar and vinegar.
Turn up the heat and caramelize the vegetables.
Taste for the correct balance of sweet and sour.
I use regular white sugar and wine vinegar.
Add the capers, stir and turn off the heat.
This is great hot, but best the next day at room temperature.

More recipe and a word of caution:
- Skillet Steak Peperonata Recipe | Leite's Culinaria
A word of caution:
Don’t use green bell peppers in this peperonata.
We know, some of you love your green bell peppers.
And that’s very nice for you.
But red, yellow, and orange bell peppers have a distinctly different flavor profile, one whose sweet notes meld marvelously with the other ingredients here.
The rather unique smack of green bell peppers? Not so much. Trust us.

- Peperonata (Sweet Bell Peppers With Olive Oil, Onion, and Tomatoes) | Serious Eats
Notes:
For the pureed tomatoes, you can use a puree of fresh peeled and seeded tomatoes that have been briefly cooked to concentrate their flavor, or canned whole tomatoes that you've blended with their juices. (Canned tomatoes are often the better choice, since they are usually top-quality.)
If you make this with beautiful, ripe summer bell peppers, they should provide enough sweetness to balance the small amount of vinegar.
If your peppers are less sweet and the peperonata tastes a little too tart, add a tiny bit of sugar, a pinch at a time, until the flavor is balanced.

- Peperoni in padella (Angelina's Fried Peppers) | Memorie di Angelina
Take it from Angelina:
Don’t be shy with the oil.
You need to use a lot of oil to get the right flavor and consistency for this dish.
Make sure that all the vegetables glisten; if not, add more oil.
And use olive oil—no other oil will do.
You can remove the excess before serving if you want a lighter dish.
Many recipes for fried peppers call for adding a bit of fresh tomato or tomato purée. I’m not partial to either of these variations, but that’s a matter of personal preference.
Whatever other ingredients you decide to add to your fried peppers, be sure that you cook the peppers until they are actually done.
The Italian way with vegetables is to cook them until they are completely tender—not crisp-tender as in Asian stir-fry or trendy North American cooking.
This is particularly important for peppers; they have a completely different taste when they are raw, which some people have described (charitably if you ask me) as ‘tangy’.

- Nigel Slater's classic peperonata recipe | Life and style | The Guardian
The trick
The peppers must be sweet and ripe.
Don't brown the onion, just let it soften to a pale golden colour.
The texture of the peppers and onions should be silky and soft enough to crush between your fingers.
The stew will mellow when left in the fridge for a day or two.
The twist
I'm not sure I'd risk incurring the late Mrs David's wrath by introducing any twists, but it is worth noting its uses beyond that of an antipasto.
I have served it on both soft and toasted polenta, used it as a pasta sauce and even stirred cooked penne through it before baking it with a few slices of mozzarella on top.
Drained of its liquid, this recipe makes a splendid stuffing for sourdough sandwiches.
You could ring the changes with tarragon instead of the basil or even coriander leaves, as long as you don't call it peperonata.
I have used the last dregs from a dish as a pizza topping with surprising success.

- Peperonata | Williams-Sonoma
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