Place a rack in the top third of the oven and preheat the oven to 400°F [200°C]. Line a baking sheet with parchment paper.
Place all the bell peppers in a medium bowl. Drizzle with 1 Tbsp of the olive oil, sprinkle with a generous pinch of salt, and toss until the peppers are well coated. Transfer to the prepared baking sheet. Roast for 15 minutes, turn the peppers, and continue roasting until the peppers are charred and soft, with their skins beginning to peel away, an additional 20 minutes.
Return the peppers to a medium bowl, cover tightly with plastic wrap, and let sit for 10 minutes. When the peppers are cool enough to handle, remove the stems, skin, and seeds. Cut the flesh into rough strips and set aside.
In a large sauté pan over medium heat, warm the remaining 4 Tbsp [60 g] olive oil. Add the onion and fennel and cook, stirring occasionally, until softened, 8 to 10 minutes. Add the garlic, crushed red pepper, and 1/2 tsp salt and cook, stirring constantly, until fragrant, about 1 minute. Stir in the capers and vinegar and allow the vinegar to reduce for 1 minute. Remove from the heat. Stir in the roasted peppers.
Transfer to an airtight container. Taste and adjust the salt and pepper as desired. If using, finish with a generous drizzle of extra-virgin olive oil.
Store in the refrigerator for up to 1 week.