Instructions:

In a saucepan over medium heat, melt the butter. Add the onion and jalapeno and cook until the onions are browned and softened, about 10 minutes.
Whisk the flour into the butter and let bubble for about 1 minute.
Slowly add the milk to the pan, whisking constantly, and simmer until the sauce thickens, about 5 minutes. Stir in the tomatoes, green chiles, and half the cilantro.
Turn the heat down to low, and add the cheese slowly, about 1/2 cup at a time, stirring into the sauce until completely melted.
When the sauce is smooth, stir in the sour cream. Taste and season with salt and pepper. Serve right away.

Note: If the sauce starts to firm up too much, put it back over low heat until melted and smooth.

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