Bring kombu and 6 cups cold water to a bare simmer in a large saucepan over low heat. Remove from heat and let sit 10 minutes. Return to a bare simmer and add bonito flakes. Remove from heat and let sit 2 minutes. Strain dashi through a fine-mesh sieve lined with 2 layers of cheesecloth into a large bowl.
Wipe out saucepan and pour in dashi. Add soy sauce; season with salt. Bring to a bare simmer over medium-low. Whisk quickly to create a vortex and gradually stream in eggs, pouring in about 1" from edge of pan. Remove from heat and add spinach; stir until wilted. Divide among bowls. Top with scallion and drizzle with chili oil. Serve with lemon wedges.
