Using an Instant Pot? Cook onions with sauté function, then add next 3 ingredients, seal and cook 10 min.
- 2 tbsp. butter
- 6 c. onions (2 lbs.) thinly sliced
- 1 tbsp. all-purpose flour
- 6 c. low-sodium beef broth divided
- 2 tbsp. Worcestershire sauce
- 1 tsp. dried thyme
- 16 (1/2"-thick) slices thin baguette toasted
- 1 c. grated Gruyère cheese
In skillet over medium-high heat, melt butter. Add onions, cook until browned, about 12 min., stirring often. Add flour; cook 1 min., stirring. Stir in 1/2 cup broth, scraping up browned bits.
Transfer mixture to 4-qt. slow cooker. Add Worcestershire, thyme, 1/2 tsp salt, 1/2 tsp. pepper, and remaining broth. Cover; cook on high 4 hrs.
Heat broiler. Line rimmed baking sheet with foil; coat with cooking spray. Arrange toasts on baking sheet. Top each with 1 tbs. cheese. Broil 1-2 min. until golden brown.
Divide soup among cups or bowls; top with toasts.