Place rack on lowest rung of oven and preheat to 375 degrees.
Score squash halves and place on rack in roasting pan. Spread squash flesh with butter. Spoon ricotta into squash hollows. Drizzle with olive oil. Season liberally with salt and pepper. Dust with nutmeg.
Bake at 375 degrees until flesh is fork tender, approx. 45-55 minutes. Remove from oven and rest 5-10 minutes before serving.