Rinse and dry the produce. Place a large pot of salted water on to boil. Peel and halve the onion. Peel the carrots. Peel 3 garlic cloves. Cut the onion, carrot, and celery into big chunks and place in a food processor with the garlic. Pulse, stopping to scrape the sides once or twice, until the vegetables are well chopped and even, but not puréed.
Place a large skillet over medium-high heat with 2 tbsp olive oil. Once hot, add the vegetables and cook, stirring frequently until they begin to soften, about 2 to 3 minutes. Add the tomato paste and cook, stirring frequently until the mixture darkens slightly; 4 to 5 minutes. Add the crushed tomatoes, lower heat and simmer, stirring occasionally until the sauce thickens, about 8 to 12 minutes.
Add the Banza shells to the large pot of boiling water and stir. Cook until al dente, about 7 to 9 minutes. When the pasta is ready, reserve 1 cup of the cooking water. Drain the pasta, and return to the large pot.
Rinse, dry and zest the lemon. In a food processor, combine the the pepitas, nutritional yeast, ¼ tsp of salt, and a pinch of pepper. Pulse to a coarse meal, transfer to a small bowl, and stir in dried oregano and 1 tsp lemon zest.
Season the bolognese with salt and pepper. Add the Banza shells and half of the reserved pasta water to the skillet. Bring pasta to a simmer, taste, and season with salt and pepper if necessary.
To serve, divide the pasta bolognese between large plates or bowls. Top with pepita parmesan and a pinch of remaining lemon zest.