Preheat oven to 375 degrees. Cut spaghetti squash in half; scoop out seeds. Brush spaghetti squash with 1/2 tablespoon olive oil and sprinkle with salt and pepper. Place squash rind-side up in a baking dish. Bake for 45 minutes or until you are able to gently pull spaghetti squash strands from the rind. Remove all of the squash from the rind; set aside.
Heat grill pan over medium heat and cook sausage 10-12 minutes on each side or until thoroughly cooked. Slice into 3/4" pieces.
Sauté 1 tablespoon olive oil and minced garlic in a pan over medium heat for 2-3 minutes or until fragrant. Add diced tomatoes to pan; continue to cook for 4-5 minutes. Add spaghetti squash to pan and continue to cook for another 4-5 minutes. Add spinach to pan and allow to wilt. Gently mix feta cheese and sausage pieces into spaghetti squash mixture.