Heat the olive oil in a 12-inch heavy skillet over medium heat until hot but not smoking.
Add the thinly sliced onions, chopped garlic, 1/4 teaspoon salt, 1/8 teaspoon black pepper, and the bay leaf; reduce heat to moderately low, cover, and cook, stirring occasionally, until the onions are very tender and golden, about 20 to 25 minutes.
Discard the bay leaf and keep the cooked onions warm in the skillet off the heat.
Meanwhile, place the French green lentils in a 1½- to 2-quart saucepan and cover with water by 1 inch; bring to a boil over medium heat, cover, and simmer until the lentils are just tender and most of the water is absorbed, about 15 to 20 minutes.
Season the lentils with a pinch of salt and pepper, then keep them covered until ready to use.
Cook the pasta in a 6- to 8-quart pot of boiling salted water until al dente, then reserve 1 cup of the cooking water and drain the pasta.
Return the drained pasta to the pot, add the cooked onions and lentils, then add the baby spinach.
Toss everything, adding just enough reserved pasta water to wilt the spinach and moisten the pasta.
Stir in the grated Parmigiano-Reggiano and adjust seasoning with salt and pepper to taste, then serve immediately.
