Make the pickled red onions:
- Place the onions in a bowl and toss with the sugar and salt. Let sit for 10 minutes, the drain off any excess liquid.
- Stir in the vinegar and dill and taste, adjusting salt and sugar. Set aside.
Make the pate:
- Add mackerel, cream cheese, mayonnaise, Dijion mustard, lemon juice, cayenne, pepper to a food processor or blender and blend until thick and creamy, adjusting lemon, and pepper as necessary.
- Spread mustard on toast, then top with pate, dill, capers, and the pickled onions.