Finely chop the onions, garlic and jalapeños.
Melt the butter in a saucepan and sauté the onions together with the garlic.
Dust the sauteed onions and garlic with flour and stir in.
Gradually add the milk and bring to a simmer, stirring.
Melt the cheese over low to medium heat, stir in the jalapeños and season to taste.