Adjust an oven rack to the top third of the oven and preheat to 400°F. Prepare a baking sheet by lining it with parchment paper.

In a medium bowl, toss the red and yellow bell peppers with 1 tablespoon of olive oil and a generous pinch of kosher salt until evenly coated.

Spread the peppers onto the prepared baking sheet. Roast for 15 minutes, then flip the peppers and continue roasting for another 20 minutes, or until the skins are charred and soft, and the peppers start to collapse.

Once roasted, transfer the peppers back to the bowl. Cover the bowl tightly with plastic wrap and let them steam for 10 minutes. When cool enough to handle, peel off the skins, remove the stems and seeds, and cut the flesh into rough strips. Set the strips aside.

Heat the remaining 4 tablespoons of olive oil in a large sauté pan over medium heat. Add the sliced onion and fennel, stirring occasionally, and cook until softened, about 8 to 10 minutes.

Add the minced garlic, crushed red pepper flakes, and ½ teaspoon of kosher salt to the pan. Stir constantly for about 1 minute until the mixture becomes fragrant.

Stir in the capers and sherry vinegar, allowing the vinegar to slightly reduce, which takes about 1 minute. Remove the pan from the heat and mix in the roasted bell peppers.

Transfer the cooked peperonata to an airtight container. Taste and adjust the seasoning with additional salt and black pepper, if necessary.

To serve, drizzle a little extra olive oil on top for added richness, if desired. This dish can be refrigerated for up to 1 week.