Preheat the oven to 425°F. In a 10-inch sauté pan, heat the olive oil over medium-high heat. Add potatoes and season well with salt and pepper. Cook for 3 minutes, lower the heat to medium-low, and add the onions. Season again, and cook until the onions are soft and caramelized, about 10-12 minutes. Stir in the sage.
Meanwhile, combine the eggs, half & half, salt, and pepper. Beat well. Whisk in the parsley, chives, and 2 tablespoons of Pecorino Romano. Pour this mixture into the pan. Stir gently, but do not let the eggs scramble. Let the eggs cook undisturbed until they set around the edges, about 3-4 minutes. Sprinkle the remaining 2 tablespoons of Pecorino Romano over the top and dollop with ricotta. Transfer the pan to the oven and cook until the eggs are completely set, about 5-6 minutes (see notes).
In a large bowl, toss the tomatoes with the olive oil, parsley, salt, and pepper.
Loosen the sides of the frittata with a spatula and slide it out onto a platter. Cut a piece and garnish with more Pecorino Romano if desired. Season to taste. Top each piece with the fresh tomatoes.
Depending on a variety of factors, the eggs may cook differently, so keep an eye on the frittata as it bakes. Insert a toothpick in the center, and if it comes out runny, keep cooking until the toothpick comes out dry.