Blackberry Farm’s Sweet and Spicy Foothills Coleslaw

3 Tbs. Dijon mustard 

1/4 cup firmly packed light brown sugar 

3 Tbs. malt vinegar 

1 1/2 tsp. mustard seed oil or very spicy mustard such as English or German 

1 1/2 tsp. kosher salt, plus more, to taste 

1/4 tsp. freshly ground pepper, plus more, to taste 

1 cup mayonnaise 

3 cups thinly sliced red cabbage 

3 cups thinly sliced green cabbage 

1 1/2 cups shredded carrots 

3/4 cup shredded red onion