Cook the pasta according to the packet instructions but on the lower end time-wise - al dente is better. Drain and set aside when ready, saving a little of the pasta cooking water just in case.
Meanwhile, peel and roughly chop the onion and garlic.
Warm part of the olive oil in a wide skillet/frying pan over a medium heat. Add the onion and garlic and cook a couple minutes until softening but take care they don't burn - reduce heat and/or add more of the oil as needed.
Add the pecans and toast briefly. Reduce the heat, if needed, and add the spinach and wilt down in the heat of the pan - take care not to burn it, you just want it to soften and wilt. Add the basil, wilt it as well, then remove from heat.
Place the onion, garlic, pecan, spinach and basil mixture in a blender and add the remaining oil, evaporated milk and crumbled queso fresco. Add a little salt and pepper, to taste (bear in mind the cheese can add a saltiness so you may want to only add a little pepper). Blend until smooth.
Return the sauce to the skillet/frying pan and warm gently for a minute as you add the cooked pasta. Toss gently to coat the pasta evenly. If it is very thick, add a little reserved cooking water to loosen it up, then serve.