Fill a bowl with ice water. Stir spinach into a large pot of boiling water, cook 20 seconds, then transfer to ice water. When cool, drain, squeeze dry, and chop.
In a 10-in. ovenproof nonstick frying pan over medium heat, cook leek in 1 tbsp. olive oil, stirring often, until very tender, 5 to 7 minutes. Stir in garlic; cook, stirring, until fragrant, about 1 minute. Add spinach and rosemary; cook briefly, stirring to coat with oil. Remove from heat.
Whisk eggs in a bowl. Beat in 1/2 tsp. salt and ricotta. Stir in cooked greens and leek. Add pepper to taste.
Preheat broiler. Clean and dry frying pan. Set over medium heat and add remaining oil. When hot, pour in egg mixture; cook, stirring occasionally to distribute cheese and greens, 1 minute. Continue cooking, tilting pan and using a spatula to lift up the frittata's outer edge and let eggs run underneath, until eggs no longer flow easily. Turn heat to low and cover pan. Cook until frittata is almost set, 6 to 10 minutes.
Broil frittata about 3 in. from heating element until just beginning to color on top, about 2 minutes; remove from oven. Let sit in pan about 5 minutes, then carefully slide out onto a platter. Allow to cool, then cut into diamonds (about 1 1/2- by 3-in., tip to tip). Add more salt to taste.
Make ahead: Up to 1 day, then chill, covered. Bring to room temperature to serve.
Note: Nutritional analysis is per serving.
Servings Serves 8 to 10 as an appetizer
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.