Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F.
• In a large heavy-duty saucepan, melt 3 Tbs. of the butter over medium-high heat. Add the onion, raise the heat to high, and cook, stirring occasionally, until beginning to brown, about 5 minutes. Add 2 Tbs. water and the baking soda, and continue to cook, stirring occasionally, until the liquid has evaporated. Add more water 2 Tbs. at a time as needed until the onions are deep golden brown, about 8 minutes. Add the vinegar, and cook for another 30 seconds. Set aside to cool.
• Meanwhile, beat the eggs in a large bowl.
• Heat 1 Tbs. of the butter and 1 Tbs. of the oil in a large nonstick skillet over medium-high heat. Add the leeks and cook, stirring occasionally, until browned in spots, about 5 minutes. Let cool briefly, then transfer the leeks to the bowl with the eggs. Add the onions, prosciutto, parsley, mustard, 1/2 tsp. salt, and 1/2 tsp. pepper, and stir to combine.
• Heat the remaining 1 Tbs. butter and 1 Tbs. oil in the same skillet over medium heat. Pour in the egg mixture; if necessary, use a fork to distribute the ingredients evenly. Lay the asparagus spears across the top. Bake until set, 15 to 17 minutes. Allow to rest for 5 minutes. Using a thin spatula, slide the frittata onto a serving dish, and cut into wedges.