Over an open flame on the stovetop, use tongs to roast the jalapenos, turning so that the skin becomes evenly charred. You can also broil them, turning once.
When the chiles are blackened, place in a bowl and cover tightly with plastic wrap. Allow to steam for about 10 minutes to loosen the skin. Remove from the bowl, peel, stem, and seed the jalapenos. Dice, and set aside.
Spray a large lidded pot with a thin film of cooking spray or with some olive oil.
Heat the pot over medium-high heat and add the spinach, cooking for 1 to 2 minutes, covered, or until the spinach wilts.
Remove the pot from the heat and place the spinach in a colander. Press out excess liquid, and place spinach in a food processor, in batches if necessary. Pulse to roughly chop the spinach, place in a bowl and set aside.
In a small saucepan over medium-low heat, melt the butter. Add the onion and sauté for about 5 minutes, or until translucent. Stir in the flour, and cook, whisking constantly, for about 2 minutes. The mixture should smell toasted.
Pour in the milk, whisking constantly until smooth. Simmer over low heat for 5 to 7 minutes, or until fairly thick, whisking frequently.