Preheat oven to 425°F. Arrange bell peppers, onions, fennel, and garlic on a large rimmed sheet pan. Drizzle with olive oil and season with salt, pepper, and Italian seasoning. Toss to combine and arrange in a single layer.
1 medium red bell pepper, 1 medium yellow bell pepper, 1 medium green bell pepper, 1 medium red onion, 1 cup thinly sliced fennel, 4 medium garlic cloves, 1 Tablespoon olive oil, ½ teaspoon kosher salt, ¼ teaspoon black pepper, 1 teaspoon Italian seasoning