In a large sauté pan over medium-high heat, put 1 tablespoon of olive oil. Add the diced onion and sauté until translucent. Add the greens and sauté until cooked down, about 3 additional minutes. Add the feta and ricotta to the greens and mix thoroughly. In a small cup, mix 1 teaspoon cornstarch with 2 tablespoons of water. Add to the greens. Continue cooking over medium-high heat until the mixture thickens. Season with salt and fresh-ground pepper to taste. Allow the mixture to cool until just barely warm.