1. Preheat the oven to 350°. Butter two 9-inch springform pans and dust with 2 tablespoons of the Parmesan cheese.
2. In a medium enameled cast-iron casserole, bring the water to a boil with a large pinch of salt. Whisk in the grits. Cover and simmer over low heat, whisking often, for 1 hour and 15 minutes. Transfer the grits to a large bowl and let cool, stirring occasionally, until warm.
3. Meanwhile, in a medium saucepan of boiling water, working in 2 batches, blanch the spinach for 30 seconds. Using a slotted spoon, transfer the spinach to a colander. Blanch the parsley in the boiling water until almost tender, about 4 minutes. Transfer to the colander. Lightly squeeze the greens dry and coarsely chop.
4. In a large skillet, melt 2 tablespoons of the softened butter. Add the garlic and cook over low heat until fragrant, about 2 minutes. Add the spinach and parsley and cook, stirring, for 2 minutes. Season with salt and pepper.
5. Stir the tetilla, Mahón and Manchego cheeses, the remaining 3 tablespoons of softened butter and the spinach and parsley into the grits. Season with salt and pepper and stir in the eggs. Pour half of the mixture into each of the prepared pans. Brush the frittatas with the melted butter and sprinkle with the remaining 2 tablespoons of Parmesan.
6. Bake the frittatas on the top shelf of the oven for about 40 minutes, or until golden brown and just set. Let cool in the pans for 15 minutes. Remove the pan rings. To serve, cut the frittatas into wedges with a serrated knife, wiping the knife between cuts.
Make Ahead: The frittatas can be refrigerated in the pans for up to 2 days. Bring to room temperature before reheating.
Notes: Stone-ground grits are available from www.hoppinjohns.com or call 800-828-4412.