Boil the fettuccine according to the package directions as you prepare the rest of the dish
Toss chicken tenderloins in cornstarch, oregano, garlic powder, salt, and pepper to evenly coat.
Heat olive oil in a large skillet over medium-high heat. Add the chicken tenderloins and brown well on both sides, cooking until a meat thermometer registers at least 165°F when inserted into the thickest piece. Remove chicken from skillet and set aside.
Reduce heat to medium. Add the sliced mushrooms to the residual oil in the skillet and sauté until browned. Stir in garlic and sauté 1 additional minute.
Pour in marsala and heavy cream and bring up to a simmer. For more flavor, stir in beef bouillon. Let simmer 3 minutes.
Stir in fresh spinach and cook 1 minute, or until wilted.
Stir in cooked fettuccine and cooked chicken tenderloins and let cook 1 minute, just until the sauce is coating the pasta. Season with salt and pepper to taste and serve immediately.