4 ingredients and 2 minutes of effort--that's all it takes to make the most amazing mustard, the kind you'd pay good money for at a fancy market. This mustard is about as easy as it gets, and there are so many ways to modify it to suit your tastes.
Any blonde ale should do the trick for this recipe. It will go exceptionally well with the breadiness of the tortillas, and the earthy notes of the portobello mushrooms. A pilsner, brown, or wheat beer would also work well with the flavor profile.