Bring a large pot of lightly salted water to a boil.
Lay chicken tenders out on a plate and pat dry. In a small bowl, mix together garlic powder, Italian seasoning, salt, and pepper. Sprinkle half of the spice mixture evenly over the chicken tenders, flip, and season with the remaining half of the spice mixture.
Heat a large skillet over medium-high heat. Add oil and butter; when the butter is melted and sizzling, arrange chicken tenders in a single layer. Cook for 4 to 5 minutes or until the bottom is nicely browned, then flip and cook for another 3 to 4 minutes or until cooked through (internal temperature in the thickest part of the largest tender should read 160 degrees F). Transfer chicken to a cutting board and cut into bite-sized pieces.
Add pasta to the pot of boiling water and set the timer for about 1 minute less than the package instructs (the pasta will finish in the sauce). For example, if your linguine package specifies 10 minutes, cook for 9 minutes until it is just barely al dente.
Return the skillet in which you cooked the chicken to the stove (no need to clean it) over medium heat. If the skillet seems dry, you can add a splash of olive oil or a pat of butter as needed.
Add onion and cook until softened, 4 to 5 minutes. Add garlic and red pepper flakes and stir for 30 seconds until fragrant. Add flour and stir until no dry bits of flour remain. Add chicken broth, a splash or two at a time, whisking constantly until smooth and all of the broth has been incorporated. Whisk in mustard.
Continue to whisk the sauce mixture over medium heat until it starts to bubble gently and thicken, about 5 minutes, then whisk in cream. Add a splash of lemon juice and salt and pepper to taste (if you used unsalted or low-sodium chicken broth, you’ll probably need ½ teaspoon of salt or more; if your broth was on the salty side already, use less). Your pasta should be just about done at this point; if not, reduce the heat on the sauce to keep it warm until the pasta is just barely al dente.
Use a large slotted spoon or colander to drain pasta and dump it straight into the saucepan with the sauce, reserving a cup or so of pasta water. Top with spinach, then gently fold to coat the pasta completely with sauce, and let it finish cooking for a minute or two until the sauce is nice and thick, coats the pasta completely, and the spinach is gently wilted. Add a splash of pasta water if needed to loosen up the sauce a bit more.
Fold in chicken until warmed through. Spoon pasta into serving bowls, top with shaved parmesan cheese; serve immediately.