Bring a large pot of salted water to a rolling boil. Add spaghetti and cook until al dente, about 8 to 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. Cook for about 3 minutes until they soften.

Add minced garlic and sauté for an additional minute. Toss in fresh spinach, stirring until it wilts down significantly, about 2 minutes.

Transfer the sautéed mixture to a blender. Add roasted poblano peppers, cilantro, sour cream, bouillon (if using), and milk. Blend until smooth.

Return the cooked spaghetti to the pot and pour the blended poblano sauce over pasta. Toss to coat evenly. Stir in softened cream cheese until melted.

Season with salt and black pepper to taste. Serve hot, garnished with chopped cilantro, crumbled cheese, and pepitas.