In a medium saucepan, bring the stock and 1 1/2 cups water to a simmer over high; reduce the heat to low and keep warm.
In a large, deep skillet, heat the oil over medium. Add the onion, season with salt and pepper and cook, stirring occasionally, until softened, about 5 minutes. Add the garlic; stir until fragrant, about 1 minute. Stir in the rice; reduce the heat to medium-low.
Add 2 cups of the stock mixture to the skillet. Simmer, stirring frequently, until the liquid is absorbed, about 8 minutes. Continue adding the stock mixture, 1 cup at a time, stirring frequently, until all the stock is absorbed, about 15 minutes.
Stir in the corn; cook until tender, about 2 minutes. Remove from heat. Working in batches, add the spinach, stirring to wilt. Stir in 1/2 cup cheese and the lemon zest and juice; season. Top with the basil and remaining 1/4 cup cheese.