Preheat oven to 350°. Bring a large pot of water to boil. Cook pasta shells in boiling water for 2 minutes less than the recommended cooking time, rinse immediately with cold water and set aside. Spread spaghetti sauce over bottom of baking dish and set aside.
Meanwhile, in a large mixing bowl, combine the white beans with the ricotta using a fork to mash the beans as you go. You want some of the beans to be just half-mashed for texture. Season with salt and pepper. Add the crushed red pepper or Fresno chili. If using jarred roasted red pepper, add that to the bean mixture. Stir well to combine all ingredients.
Heat a medium skillet over med-low heat. Add 1 tbsp olive oil and 1tbsp butter to pan and heat until butter is just melted. Add leeks and garlic. Saute 2-3 minutes until fragrant. Transfer to the bean mixture.
Place skillet back on the stove and increase heat to medium-high. If you are using zucchini, add the zucchini to the skillet and cook for 4-5 minutes until just starting to get tender. Transfer to the bean mixture.
Put skillet back on stove. Add 1 tbsp olive oil and allow to heat for 30 seconds. Add 2 cups roughly chopped kale and toss until coated. Season with salt and pepper. Continue cooking 1-2 minutes until just starting to wilt (2-3 minutes for dinosaur kale). Should be al dente. Add to bean mixture.
Stuff shells with about 2 spoonfuls of bean mixture and place in baking dish. Adjust the amount of filling to ensure you have enough to fill shells for the number of portions you are preparng, 3-4 shells per person. Tear the provolone slices into pieces and place on top of shells, distributing evenly.
Bake uncovered 20-25 minutes or until provolone just starts to turn golden. Remove from oven and serve.