Heat over 450°F. Line a baking sheet with parchment paper of aluminum foil.
Peel and cut onion into thin half moons. Melt butter in skillet over medium high heat. Add onions, and pinch of salt. Lower heat to medium and cook onions for 20-30 min, stirring occasionally, cook until very dark brown.
Chop sausage into small crumbles, and cook over medium-high heat until browned and beginning to crisp. About 10 min.
Chop sweet potatoes into 1” pieces. Mince garlic and rosemary leaves. Toss sweet potatoes, garlic, and rosemary leaves in a large bowl. Add olive oil, salt and pepper, and thoroughly combine.
Add onions, and sausage to sweet potatoes, and toss.
Spread out sweet potatoes evenly on baking sheet, and bake until soft and brown.