Fill a large pot with lightly salted water and bring to a rolling boil. Stir in bucatini and return to a boil. Cook pasta uncovered, stirring occasionally, until firm to the bite and almost tender, 9 to 11 minutes. Drain, reserving 1/4 cup pasta water. Run cold water over pasta.
Heat 1 tablespoon oil in a pot over medium heat. Add 2 garlic cloves, Italian seasoning, and bay leaf. Cook and stir until garlic is fragrant, about 1 minute. Add tomatoes and mash with with a potato masher to break down. Stir and reduce heat to medium-low; let simmer.
Heat remaining olive oil in a Dutch over medium-low heat. Add sliced fennel, leeks, a pinch of salt, and remaining garlic. Cook and stir until tender, 5 to 7 minutes. Add red pepper flakes and another pinch of salt. Add the tomato sauce and reserved pasta water; stir to combine. Add the pasta and let simmer until heated through, 2 to 4 minutes. Season with salt and pepper.
Place pasta in bowls and garnish with fennel fronds and Parmesan cheese.