Set a rack in the top third of the oven. Preheat the oven to 300°F. Whisk the eggs, 1½ teaspoons salt, and the pepper in a large bowl until beaten. Stir in the cottage cheese, 1 cup of the cheese, dill, and scallions. Heat 2 tablespoons each of butter and olive oil over medium heat in the Chrissy's Fave Stainless Steel skillet. Add the onions and cook, stirring, until softened, 5 minutes. Add the garlic and cook, stirring, 1 minute.
Add the tomatoes and mushrooms along with the remaining ½ teaspoon salt and cook, stirring, until the mushrooms release their water, 5 minutes. Add the remaining 2 tablespoons each of butter and olive oil to the skillet, swirl to cover the bottom of the skillet, give the egg mixture a stir and pour it into the skillet, tilting to evenly distribute the mixture. Cook, moving the pan every minute or two to avoid burnt spots, 5 minutes. Nestle the asparagus spears on the top of the frittata, allowing the tips to peek out. Carefully Transfer to the oven and cook until the outside of the frittata is set but the center is still runny, 9-10 minutes.
Use oven mitts to remove the frittata from the oven, then raise the oven heat to broil and preheat for a few minutes. Sprinkle the remaining ½ cup cheese on top, return the frittata to the oven and broil until the center is set and the cheese is bubbly, 3 minutes. Remove from the oven and cool. Either serve directly from the pan, or invert onto a platter, then cover with another platter and invert again so the asparagus side is on top. Slice into wedges and serve warm or at room temperature.
