Begin your culinary journey by warming a tablespoon of olive oil in a spacious skillet over medium heat. Introduce the finely chopped yellow onion, stirring gently until it softens, a process that typically takes about five minutes. Next, incorporate three tablespoons of tomato paste, stirring continuously as it cooks, allowing its fragrance to emerge and its color to deepen over one to two minutes. Follow this by adding the minced garlic, Italian seasoning, half a teaspoon of salt, and a quarter teaspoon of crushed red pepper. Stir these ingredients frequently for approximately thirty seconds until their aromas are fully released.
Into the fragrant base, add the crushed tomatoes and the rinsed white beans. Bring the mixture to a gentle simmer over medium heat, stirring occasionally. Allow it to cook for about three minutes, or until the sauce slightly thickens. Carefully fold in the baby spinach, stirring constantly until it achieves a vibrant green hue and is just wilted, which should take two to three minutes. Remove the skillet from the heat, and stir in the remaining two tablespoons of olive oil. Finish by sprinkling the remaining eighth teaspoon of salt over the dish.
Divide the bean mixture evenly among four serving bowls. Garnish each portion with shaved and crumbled Parmesan cheese. For those who enjoy an extra kick, a sprinkle of additional crushed red pepper can be added. Serve this hearty skillet alongside toasted slices of whole-wheat baguette for a complete and satisfying meal.