Bring a medium sauce pot of generously salted water to a boil for the spaghetti. 

• Cut or tear the ciabatta into 1-inch pieces.

In a large frying pan over medium heat, warm 1 teaspoon [2 tsp] oil until hot but not smoking. Add the pancetta and cook, stirring occasionally, until lightly browned, 2 to 3 minutes. Spoon off all but 1 teaspoon [2 tsp] fat from the pan. Working in batches if needed, add the ciabatta and cook, stirring occasionally, until the pancetta starts to crisp and the ciabatta is lightly browned and crisp, 3 to 4 minutes. Transfer to a paper-towel-lined plate. Do not clean the pan.

While the pancetta and ciabatta are cooking, prepare the spaghetti.

Separate the spaghetti so the noodles don’t clump together during cooking. To the pot of boiling water, stir in the spaghetti and cook, stirring occasionally, until just tender, 3 to 5 minutes. Drain the spaghetti, reserving ½ cup [1 cup] pasta cooking water. Return the spaghetti to the pot; toss with about 1 tablespoon oil to keep the noodles from sticking. 

While the spaghetti is cooking, start preparing the seafood.

Pat the seafood dry on a paper-towel-lined plate (if using shrimp or scallops, rinse and drain, then pat dry); season lightly with salt and pepper (season the scallops lightly with salt).

In the same pan used for the pancetta and ciabatta, melt the butter over medium-high heat (high heat for the scallops) until hot but not smoking. 

Scallops: Add the scallops and cook without turning until golden brown on one side and starting to turn opaque, 2 to 4 minutes. Turn and continue cooking for about 30 seconds for translucent or 1 to 2 minutes for opaque.

Jumbo shrimp: Add the shrimp and cook, turning once, until firm and cooked through, 2 to 3 minutes per side. 

Salmon: Add the salmon and cook, turning as needed, until the flesh is opaque and flaky, 3 to 4 minutes. 

Transfer the seafood to a plate. Do not clean the pan. 

While the seafood is cooking, prepare the lemon.

• Zest and juice the lemon, keeping the zest and juice separate. [Zest and juice 1 lemon; save the remaining lemon for another use.]
• Measure ¼ cup [½ cup] milk; save the rest for another use.

In the same pan used for the seafood, add the reserved pasta cooking water and 2 teaspoons [4 tsp] lemon juice and bring to a simmer over medium-high heat, scraping up any browned bits from the bottom of the pan. Add ¼ cup [½ cup] milk and cook, stirring occasionally, until beginning to thicken, 2 to 3 minutes. Remove from the heat and stir in the Parmesan until melted. Stir in the spinach and 1 teaspoon [2 tsp] lemon zest.

• On a cutting board, coarsely chop the pancetta and ciabatta croutons.

To the pan with the lemon-Parmesan sauce, add the spaghetti and toss to coat. Season to taste with salt and pepper.

Transfer the spaghetti and lemon-Parmesan sauce to individual bowls. Top with the seafood and pancetta and ciabatta croutons. Garnish with the remaining lemon zest and juice if desired, and serve.