Preheat oven to 400 degrees. In a shallow baking pan arrange squash, onion, and garlic, in 1 layer and drizzle with oil, tossing to coat. Roast mixture in middle of oven 15 minutes, or until garlic is tender, and transfer garlic to a work surface. Roast squash and onion until tender, about 15 minutes more. Discard peels from squash, onion, and garlic. In a food processor puree squash, onion, and garlic with salt and pepper to taste until smooth. Squash puree may be made 2 days ahead and chilled, covered. Spread about 1/4 squash puree on each of 4 tortillas and sprinkle each with about 1/4 bell pepper and about 1/4 cheese. Top each quesadilla with a plain tortilla, pressing gently together. Spread each side of quesadilla with 1/2 tablespoon butter. Heat a griddle or 7-inch non-stick skillet over moderately high heat until hot and cook the quesadillas, 1 at a time, until golden, about 3 minutes on each side, transferring to a cutting board. Cut each quesadilla into 6 or 8 wedges and serve warm with chipolte dip. Yield: 24 to 32 hor d'oeuvres.
For the Chipotle Lime Sour Cream Dip: In a small bowl stir chili and lime juice into sour cream until combined well. Dip can be made 2 days ahead and chilled, covered.