Preparation

• Position a rack about 6 inches from the broiler and heat the broiler on high.
• 将洋葱、醋和1/2茶匙。盐在small bowl; let sit for 10 minutes and then drain and pat the onion dry. Set aside.
• Meanwhile, on a large rimmed baking sheet, toss the cauliflower with 2 tsp. of the oil, 1/2 tsp. salt, and 1/4 tsp. pepper. Broil, tossing once or twice, until the edges are golden, 3 to 6 minutes.
• Reposition the rack in the center of the oven and set the oven to 400°F.
• Whisk the eggs, dill, mustard, 1/2 tsp. salt, and 1/2 tsp. pepper in a medium bowl.
• Heat the remaining 2 Tbs. oil and the butter in a 12-inch ovenproof skillet over medium-high heat until the butter melts. Add the onion and cook, stirring occasionally, until some of the pieces are dark golden brown, about 3minutes. Remove the skillet from the heat, stir in the roasted cauliflower, and then slowly pour in the egg mixture, redistributing the vegetables evenly. Sprinkle the goat cheese on top and bake until the eggs are set in the center, about 10 minutes. Let rest for 5 minutes and then use a silicone spatula to slide the frittata onto a serving plate or cutting board. Slice into wedges and serve.