This hearty dish is equally satisfying for breakfast, lunch, or dinner. Cook an extra egg and use the hard-cooked yolk in the salad dressing for Bread and Shrimp Skewers with Romaine Salad.
Preheat oven to 450 degrees. In a large ovenproof skillet, cook bacon over medium until crisp and browned on both sides, 7 minutes; drain on paper towels. Pour off all but 1 tablespoon fat from skillet. Season onions with salt and pepper and cook, cut side down, until golden, 5 to 7 minutes. Flip, add broth, and transfer skillet to oven. Bake until onions are tender when pierced with a knife, 15 minutes.
Meanwhile, crumble bacon into small pieces. Drizzle each toast with 1/2 teaspoon oil and spread with mustard. Top each toast with 1 egg, season with salt and pepper, and sprinkle with bacon. Divide toasts and onions among 4 plates. Place skillet over medium-high and add 1 teaspoon oil and garlic; cook until fragrant, 30 seconds. Gradually add spinach and cook, tossing, until wilted. Season spinach to taste with salt and pepper and serve with toasts and onions.
To omit the bacon, cook the onions in 1 tablespoon olive oil. Sprinkle the finished toasts with chopped fresh herbs.