We heat up our 10 inch non-stick pan (a pan that is oven-safe) with the olive oil over medium heat. We add the sliced onions and turn down the heat to medium-low. We cook stirring occasionally until softened and golden.
Meanwhile, we beat the eggs, cream cheese, horse radish, and salt and pepper in a medium bowl with a fork. When the onions are ready, we pour the egg mixture into the pan and raise the heat to medium. We sprinkle the tomatoes on top and cook for a few minutes until the bottom is brown. We then place the pan in the oven and cook for about 10 minutes. We want the frittata to be cooked through.
We carefully take the pan out of the oven with a potholder and slide the frittata onto a plate. We flip it over using a second plate and slide it back into the pan. We want to brown the other side of the frittata so we place the pan on the stove again and heat on medium heat until browned. We then flip the frittata onto a serving plate and enjoy with capers sprinkled on top!