In a large pot, bring 4 quarts of water to a boil. Add the pasta and cook according to package directions. Meanwhile, heat a large skillet over medium-high heat. Spray with nonostick vegetable coating. When the skillet gets very hot, add the mushrooms, spreading out thinly. Cook for 1-2 minutes to brown. Toss and cook for 2 minutes more. You want them to brown and not get watery. Turn off the heat and reserve until the pasta is cooked. When the pasta is al dente, drain in a colander and return to the cooking pot. Toss with the olive oil. Add the spinach, garlic, stock, olives, and the mushrooms. Toss well to mix. Place on individual plates or a serving platter and top with feta cheese and the almonds.