In a medium saucepan, combine squash, garlic, milk and broth; bring to a boil over medium-high heat. Reduce heat to medium-low; partially cover and simmer until squash is tender when pierced, about 12 to 15 minutes. Remove from heat; stir in salt, mustard, cayenne and nutmeg.
Using an immersion blender, blend squash mixture in pot until smooth. (Alternatively, let cool slightly. Transfer to blender; place lid, without its centre feeder cap, on blender. Cover blender lid with a kitchen towel and, holding towel and lid securely, carefully blend on low speed until smooth. Don't fill blender more than half full; blend in batches, if necessary.)
Meanwhile, cook pasta in a large pot of boiling salted water, according to package directions, until tender but still firm. Drain and transfer to a large bowl; stir in squash mixture, the ricotta and 1¼ cups (300 mL) of the Cheddar cheese. (The mixture will seem soupy but thicken when baked.)
Scrape mixture into a lightly greased 10-cup (2.5 L) ceramic oval or 9-inch (2.5 L) square ceramic or glass baking dish. Toss remaining ½ cup (125 mL) of the Cheddar cheese and bread crumbs with oil; sprinkle evenly on top.
Place on a baking sheet and bake in a preheated 400°F (200°C) oven until bubbly and golden, about 20 minutes. Let stand 5 minutes before serving.
Nutrients per serving: 395 calories, 20 g protein, 15 g fat (8 g saturated fat), 52 g carbohydrates (6 g fibre), 39 mg cholesterol, 583 mg sodium