Bring a large pot of salted water to a rolling boil. Add the dry spaghetti and cook according to package directions until al dente, about 8-10 minutes. Reserve 1 cup of pasta water before draining.

In a large skillet, heat olive oil over medium heat. Add diced onion and chopped jalapeño or serrano pepper. Sauté for about 3 minutes until the onion becomes translucent, then add minced garlic and spinach. Cook for an additional 2 minutes.

Transfer the sautéed mixture into a blender along with roasted poblano peppers, fresh cilantro, sour cream, vegetable bouillon, and milk. Blend until smooth. Adjust thickness with more milk if necessary.

Pour the blended sauce back into the pot over medium-low heat. Gradually stir in cream cheese until melted. Season with salt and black pepper.

Add the drained spaghetti back into the pot. Toss gently to coat the pasta in the sauce. If needed, add reserved pasta water to achieve desired creaminess.

Serve hot, garnished with chopped cilantro, crumbled cotija cheese, or toasted pepitas.