Preheat the oven to 375°F.
Heat a drizzle of oil in a 10-inch ovenproof skillet set over medium heat. Sauté the asparagus for 2-3 minutes, then add the tomatoes and garlic to the pan and cook for another 2 minutes, until the tomatoes release their juices. Add the spinach and cook until it wilts.
Meanwhile, stir together the eggs, cheese, salt and pepper in a medium bowl. Spread the vegetables into an even layer in the pan and pour the beaten eggs overtop; reduce heat to medium-low and cook the frittata for 5-8 minutes, until the bottom is set. To help it along, gently pull back the sides with a heatproof spatula to allow any uncooked egg to run underneath.
Transfer the skillet to the oven and bake for about 10 minutes, until the top is set and golden. Serve hot, at room temperature, or cold.
Serving Size Serves 4.