Heat half the oil in a 12-inch skillet over medium-high heat until it shimmers. Add the fennel and cook until softened and lightly browned, 5 to 7 minutes.

Stir in the garlic, orange zest, fennel seeds, pepper flakes, and saffron. Cook until fragrant, about 30 seconds, then stir in the tomatoes. Simmer until the tomato pieces lose their shape and form a chunky sauce, about 10 minutes, then discard the orange zest.

Bring the water to a boil in a large pot. Add the pasta and salt, stirring often, and cook until al dente. Reserve a small amount of the cooking water, then drain the pasta and return it to the pot.

Stir the orange juice, basil, salt, and pepper into the sauce, adding sugar to taste if needed. Add the sauce and the remaining oil to the pasta and toss to combine, adding the reserved cooking water as needed to reach a glossy consistency. Serve warm.