reheat the oven to 425°F. Line a baking sheet with foil.
Toss the peppers with olive oil and arrange in a single layer. Roast for 10–12 minutes, flipping halfway through, until blistered and lightly charred in spots. Let cool slightly, then remove the stems and finely chop. You should have about 1/4 cup chopped peppers. Set any extra aside for garnish.
Reduce the oven temperature to 350°F.
In a medium bowl, stir together the softened cream cheese, mayo, cheddar, salt, and pepper until smooth. Fold in the chopped peppers. Adjust seasoning to taste.
Transfer the mixture to a small baking dish and smooth the top. Sprinkle the remaining 1 tablespoon of cheddar over the surface.
Bake uncovered for 10 minutes, until the dip is hot and the cheese is melted and slightly golden around the edges.
Finish and Serve. Top with the reserved whole or chopped roasted peppers and serve warm.