Tarragon and garlic mustard

Mustards are easy to make. You can buy many striking ones when you’re on holidays in country towns, cute jars with gingham cloth covers and handwritten labels … rhubarb, quince and chocko mustard … or whatever. Inventive combos, and they may sit in the cupboard and desiccate for a few years before you chuck them. I’ve come to like purpose-built mustard. I know what I like it with (croque monsieur