3/4 cup yellow mustard seeds
1/2 cup brown mustard seeds
1/2 teaspoon fenugreek seeds
3 cups rhubarb cut into 1/2-inch pieces
1/2 cup sugar
Just under 1 3/4 cup vinegar (cider vinegar, mostly likely)
1/2 teaspoon salt
Grind the mustard and fenugreek seeds into a fine powder, leaving some portion of the yellow mustard seeds whole. Put everything in a wide pot and cook over medium heat until the rhubarb is helplessly soft.
Run it though the coarse disc of a food mill if you like things smooth rather than chunky. Process 15 minutes in a boiling water bath if you want to keep it on the shelf.