A mustard sauce with caramelized onions sounds great.
I've done brat pizza before, but with regular brats, the hatch chilis change the equation. In our case, we did brats, sauerkraut, and onions (not caramelized) with a traditional red sauce. It was good, but to my recollection, a brat is actually a milder flavor than an Italian sausage or a pepperoni.
Both versions turned out well but I preferred the garlic sauce and black shallot over the mustard sauce and sauerkraut.
Some friends stopped by with some dough and prosciutto from the market so I made a second pizza. I added some black shallots and Thai basil to their pizza. They are both adverse to garlic so they did not try the garlic sauce but thought the mustard sauce was good. I thought their pizza was good too.
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