1. Coat steaks with 1 tbsp of olive oil and season with salt and pepper to taste. Allow to rest at room temperature while preparing noodles.
2. Heat 1 tbsp of olive oil in a saucepan over medium-high heat. Add onions and cook for about 3 minutes, until onions are soft and translucent. Remove from heat and cool down.
3. Place cooled onion mixture in a food processor and add garlic, baby spinach, basil, evaporated milk, walnut halves, lime juice, and queso fresco. Process to a thick purée. Taste and adjust seasoning.
4. Fill pot with water and bring to boil. Cook fettuccini per package instructions. Drain, saving ¼ cup of cooking water.
5. Transfer green mixture to pot, add cooked fettuccini and 1 tbsp reserved cooking water (or more to reach desired consistency). Gently stir to coat pasta. You may add additional cooking water to reach desired consistency.
6. Heat cast-iron grill over medium-high heat. Add steaks to grill and cook until golden brown on all sides, about 3 - 4 minutes per side for medium-rare to medium. Transfer steak to a cutting board and slice into strips, approximately ½” wide.
7. Portion pasta on plates and top with steak strips. Garnish with fresh basil and crumbled queso fresco. You may sprinkle a pinch of red pepper flakes over each plate (optional.)