Heat half the oil in a 12-inch skillet over medium-high heat until the oil should shimmer but not smoke. Add the fennel pieces and cook until they are softened and show light brown edges, about 5 to 7 minutes.

Stir in the garlic, orange zest, fennel seeds, pepper flakes, and saffron. Cook until the aromatics are highly fragrant, about 30 seconds, keeping them moving so the garlic does not burn.

Add the tomatoes and cook, stirring occasionally, until the pieces lose their shape and break down into a chunky sauce, about 10 minutes. Once the sauce has thickened, remove and discard the orange zest strips.

While the sauce simmers, bring 4 quarts of water to a boil in a large pot. Add the pasta and salt and cook, stirring often, until al dente. Reserve 1/2 cup of the cooking water, then drain the pasta and return it to the pot.

Stir the orange juice and basil into the skillet with the sauce. Season with salt, pepper, and sugar to taste; the sugar helps balance the acidity of the tomatoes and citrus.

Add the sauce and the remaining oil to the pasta and toss to combine. Add the reserved cooking water a splash at a time as needed until the sauce coats the pasta evenly and looks glossy. Serve warm.