Step 1

Heat oven to 300°F. Position a rack near the top of the oven.

Step 2

In a heavy, well-seasoned cast-iron or nonstick, oven-proof skillet (preferably 8- or 9-inch), heat the butter over medium heat. Add onions and 1/4 teaspoon salt; cook until tender and browned in spots, about 7 minutes.

Step 3

While the onion is cooking, in a large bowl, whisk eggs; stir in 1/4 teaspoon salt and 1/4 teaspoon black pepper. Add the egg mixture to the pan, stirring gently to incorporate onions. Fold in salmon. Spoon dollops of ricotta over top of frittata; sprinkle with chives. Cook until the bottom is just set, about 2 minutes.

Step 4

Transfer the skillet to the oven; bake until eggs are set around the edges but still slightly custardy in the center, 12-15 minutes for a 9-inch skillet, 15-20 minutes for an 8-inch skillet (check frequently towards the end to avoid overcooking eggs). Slide frittata out onto a plate. Before serving, drizzle with oil and lemon, sprinkle with flaky salt and additional black pepper.