Bring a large pot of salted water to a boil and cook fettuccine for 2 minutes less than the package instructions for al dente. Drain and reserve 1/2 cup pasta water.

Mix 1/4 cup flour and 1/4 teaspoon salt in a bowl. Coat chicken in flour mixture and shake off excess.

Heat olive oil and butter in a large skillet over medium-high heat. Cook chicken tenders in a single layer until browned and the internal temperature reaches 165°F (73°C), about 8 minutes.

Add shallot to the skillet and cook until translucent, about 1 minute. Stir in mushrooms and sauté until browned, about 3 minutes.

Pour in Marsala wine and cook until slightly reduced, scraping browned bits from the bottom of the skillet, about 4 minutes.

Add chicken stock, heavy cream, Italian seasoning, black pepper, and garlic powder. Bring to a boil, reduce heat, and cook until slightly thickened, about 3 minutes.

Whisk in remaining flour and salt until smooth. Stir in spinach and cook until wilted, about 1 minute.

Add cooked fettuccine and chicken to the skillet. Toss to combine and cook over medium-high heat until heated through, about 2 minutes.