Bring a large pot of salted water to a boil over high. Add fettuccine; cook 2 minutes less than package instructions for al dente. Drain, reserving 1/2 cup cooking water.

While pasta cooks, stir together 1/4 cup of the flour and 1/4 teaspoon of the salt in a large bowl. Add chicken tenders, one at a time, and toss to coat evenly with flour. Shake excess flour off chicken, and transfer to a plate.

Heat a large skillet over medium-high, add oil and butter; cook, swirling often, until butter melts, 30 to 45 seconds. Place chicken tenders in skillet in a single layer, and cook, turning once halfway through, until browned and a thermometer inserted into the thickest part of the tender registers 165 degrees F (73 degrees C), about 8 minutes. Transfer chicken to a wire rack.

Add shallot to skillet; cook over medium-high, stirring constantly, until translucent, about 1 minute. Add mushrooms; cook, stirring often, until browned on edges, about 3 minutes.

Stir in Marsala; cook, stirring occasionally and scraping browned bits off bottom of skillet, until slightly reduced, about 4 minutes.

Stir in chicken stock, cream, Italian seasoning, black pepper, and garlic powder; bring to a boil over medium-high. Reduce heat to medium, and cook, stirring occasionally, until sauce is slightly thickened, about 3 minutes.

Whisk in remaining 2 tablespoons flour and 1 1/4 teaspoons salt until smooth, about 1 minute. Stir in spinach; cook, stirring often, until spinach is wilted, about 1 minute.

Add pasta and chicken tenders; cook over medium-high, tossing constantly, until heated through, about 2 minutes. Remove from heat. Stir in Parmesan until combined, adding reserved pasta water 2 tablespoons at a time, if needed, to reach desired consistency. Garnish with chopped fresh parsley and serve immediately.