Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the fettuccine, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 8 minutes. Drain well in a colander set in the sink.

Brown the bacon in a large skillet over medium heat until crisp, about 5 minutes; stir occasionally. Drain all but 1 tablespoon of the drippings; pour the olive oil into the skillet. Stir in garlic and onion, and cook until the onion becomes translucent, about 5 minutes. Stir in the shrimp, and cook just until pink and opaque, 2 to 3 more minutes. Mix in Italian seasoning, black pepper, and diced tomatoes, and cook, stirring occasionally, until the liquid has reduced by half, about 10 minutes.

With a spoon, move the shrimp to the outside of the skillet. Place the spinach in the center of the skillet, cover, and cook until just wilted, 3 to 5 minutes. Stir the spinach into the shrimp. Mix in the tomato paste if you prefer a thicker sauce. Serve over cooked fettuccine.