Step 1: Peel the Guisquils under running water to avoid the sticky sap. Cut them in half lengthwise and boil in salted water for about 15-20 minutes until tender but still firm.
Step 2: Once cooled, use a spoon to carefully scoop out the seed and enough of the pulp to create a cavity for the cheese. Set the pulp aside or discard.
Step 3: Fill each cavity with a generous amount of crumbled Queso Fresco. Press the cheese down firmly so it stays in place.
Step 4: In a small bowl, beat the egg whites until stiff peaks form, then gently fold in the yolks and a pinch of salt to create a light batter.
Step 5: Dredge the stuffed squash in flour, then dip into the egg batter. Carefully place in a skillet with hot vegetable oil and fry until golden on all sides.
Step 6: In a separate pot, sauté the onion and garlic, then add the tomato sauce. Simmer for 5 minutes.
Step 7: Place the fried Guisquils into the simmering tomato sauce, cover, and cook for another 5-10 minutes. Garnish with cilantro before serving.