Heat 2 tsp of olive oil in a Dutch oven over medium heat. Add in the breadcrumbs and garlic, and a pinch each of salt and pepper. Stir until combined and crumbly, mix with the parsley and then set aside.
Return the pot to the low heat, add in a teaspoon of oil and then add in the onions. Sauté over low heat until caramelized and golden, about 45-50 minutes. Add in the zucchini and cook 2 minutes, until lightly golden. Add in garlic and cook for another 30 seconds. Add in the herbs, bay leaf, thyme, and the tomatoes with their juice, and season with a pinch each of salt and pepper.
Bring the mixture to a boil. Reduce the heat to medium low, add in the white beans and balsamic, and simmer for about 15-20 minutes, until the sauce thickens.
Meanwhile, bring the polenta water to a boil in a small saucepot. While whisking gently, pour in the polenta in a steady stream and whisk until thickened. Reduce the heat to low for about 30 minutes, whisking very hard every 5-10 minutes to make sure you get all the bits off the corners and bottom of the pan. Add in the olive oil and season with salt and pepper to taste.
Five minutes before you're ready to serve, stir the spinach leaves into the cassoulet until wilted. Then divide the polenta between four bowls, top with the bean cassoulet and a generous sprinkling of garlic breadcrumbs.