6 oz. slab bacon, cut into ¼″ matchsticks
4 tbsp. unsalted butter
2 medium yellow onions, thinly sliced lengthwise
Kosher salt and freshly ground black pepper, to taste
1 ½ cups flour
2 tsp. dry mustard
1 ¼ cups milk
3 eggs, lightly beaten
1. Heat bacon in a 12″ skillet over medium-high heat, and cook, stirring occasionally, until fat renders and bacon is crisp, about 12 minutes. Using a slotted spoon, transfer bacon to paper towels to drain; pour bacon fat into a 9″ × 11″ baking dish and set aside. Return skillet to medium-high heat, and add butter; add onions, salt, and pepper, and cook, stirring, until lightly caramelized, about 10 minutes. Remove from heat and set aside.
2. Heat oven to 425°. In a large bowl, whisk together flour, mustard, and pepper; add milk and eggs, and stir until smooth. Let batter rest for 10 minutes. Meanwhile, place baking dish with bacon fat in oven and let heat for 10 minutes. Remove baking dish from oven, pour in batter, and spinkle with rendered bacon and caramelized onions; return to oven and bake until puffed and golden brown, about 30 minutes.