Add Brussels sprouts and red onions to a large mixing bowl. Pour 2 Tbsp butter over top as well as 1 Tbsp lemon juice, toss while seasoning with salt to taste (about 1/4 tsp).
Transfer to baking sheet and spread into an even layer (just around Brussels sprouts and onions).
Roast in preheated oven, tossing once halfway through baking, until Brussels sprouts have browned slightly and squash is soft, while sprinkling pecans and cranberries over top during the last 3 minutes of baking, about 40 minutes total. Serve warm.