How to Make It
Preheat a large cast-iron skillet or griddle. Add the tomatoes, garlic cloves and jalapeños and roast over high heat, turning occasionally, until softened and charred in spots, about 12 minutes. Transfer to a plate to cool. Remove the stems and seeds from the jalapeños and add them to a blender along with the tomatoes. Peel the garlic and add it to the blender. Pulse to form a coarse salsa.
Preheat the oven to 325°. In a medium saucepan, heat 1 tablespoon of the olive oil until shimmering. Add the onion and cook over moderate heat, stirring occasionally, until golden, about 5 minutes. Add the salsa and simmer over moderately low heat for 15 minutes, stirring occasionally. Season with salt and pepper and keep warm.
Meanwhile, in a large ovenproof nonstick or cast-iron skillet, heat the remaining 1 tablespoon of olive oil until shimmering. Add the spinach and cook over moderate heat, tossing, until just wilted, about 2 minutes. Pour the egg whites over the spinach and season with salt and pepper. Transfer the skillet to the oven and bake the egg whites for 12 to 15 minutes, until just set. Serve the frittata with the salsa.