Preheat oven to 425°F. Arrange squash, cut-sides up, and sweet potatoes on a baking sheet and roast in oven until fork tender, about 1 hour. Scoop out flesh from squash and sweet potatoes.

In a large pot melt butter over moderate heat. Add onion and garlic and cook until wilted, about 5 minutes. Add roasted squash, sweet potatoes, stock, salt and pepper and cook 20 minutes. Transfer to a blender and puree, being careful of the hot liquid. Transfer soup back to pot if not serving immediately, and keep warm.

Adjust seasoning. Serve soup garnished with bacon and diced potatoes.