Remove seeds and blossoms from loquats, cut in half, and place into a saucepan. Drizzle with lemon juice immediately to prevent fruit from turning brown. Cover skillet and heat over low-medium heat until loquats release their juices and come to a simmer, 12 to 15 minutes. Stir often.

Whisk together sugar and cornstarch in a bowl and pour over simmering loquats. Stir well until thoroughly combined and return to a simmer over low heat. Cook until filling has thickened, about 2 minutes. Remove from heat and let cool.

Preheat the oven to 400 degrees F (200 degrees C).

Heat a 9-inch cast-iron skillet in the oven for 5 minutes. Remove from oven and lightly coat with cooking spray. Transfer loquat filling to the skillet.

Lightly spoon flour into a dry measuring cup and level with a knife. Combine flour, brown sugar, vanilla extract, cinnamon, and salt in a bowl; cut in butter with a pastry blender until mixture resembles a coarse meal. Sprinkle mixture over loquat filling.

Place skillet on a sheet pan and bake in the preheated oven until filling is thick and bubbly and topping is browned, about 25 minutes. Remove from oven; let stand 20 minutes. Serve warm.