Bring a medium sauce pot of generously salted water to a boil. Add the spaghetti and cook until just tender, 10 to 12 minutes. Drain and set aside.
While the spaghetti cooks, make the sauce.

• Peel and coarsely chop the yellow onion.
• Trim the root from the fennel bulb; cut the fennel bulb in half lengthwise and cut away the core; coarsely chop the halves.

In a large sauce pot over medium heat, warm 2 tablespoons oil until hot but not smoking. Stir in the fennel seeds and cook until fragrant, about 10 seconds. Add the onion, fennel, and as many chile flakes as you like and season with salt and pepper. Cook, stirring occasionally, until the onion and fennel soften and start to brown, 8 to 10 minutes.
While the onion and fennel cook, prepare the remaining ingredients.

• Rinse the shrimp, then drain on a paper-towel-lined plate and pat dry. Season generously with salt and pepper.
• Cut the olives in half.
• Strip the parsley leaves from the stems; coarsely chop the leaves.

Add the tomatoes, olives, and ¼ cup white wine or water to the sauce. Bring to a simmer and cook, stirring occasionally, until the sauce thickens, 4 to 6 minutes. Add the shrimp and cook until just firm and cooked through, 4 to 5 minutes. Season to taste with salt and pepper.
Add the spaghetti and parsley to the sauce, drizzle with oil, and toss to coat.

Transfer the spaghetti, shrimp, and sauce to individual bowls and serve.

Chef’s Tip In Step 2, you can thinly slice the onion and fennel lengthwise instead of coarsely chopping them. For an elegant presentation, slice them just under ¼ inch thick so that they cook down to about the same width as the spaghetti strands.