Preheat oven to 400°F. Coat a 12-cup muffin tin with cooking spray. Heat 1 teaspoon oil in a small skillet over medium heat. Add ½ cup onion; cook, stirring occasionally, until translucent and lightly browned, about 3 minutes.
Working in batches of 5 cups, pulse 2 (10-ounce) packages spinach in a food processor until finely chopped, stopping to scrape down the sides as needed, 3 to 5 pulses. Transfer to a large bowl; do not clean the food processor bowl. Transfer the cooked onion to the food processor. Add ¾ cup feta, ¼ cup ricotta, 2 eggs, ¼ cup each dill and mint, 1 tablespoon garlic and ¼ teaspoon each pepper, crushed red pepper and salt; process until just combined, about 10 seconds. Add the feta mixture to the bowl with the spinach; stir until well combined.
Divide the spinach mixture among the prepared muffin cups (about ⅓ cup each), pressing the mixture into each cup.
Bake until the cakes are set and the edges pull away from the pan, 24 to 26 minutes. Let stand in the pan for 5 minutes. Loosen the edges with a knife and transfer to a large plate. Serve warm.
