Place the toasted bread on a cutting board and overlap slices of avocado on each piece of bread, using a quarter to half an avocado for each, depending on the size of the avocado and the bread. Sprinkle the avocado with lemon juice, then sprinkle with salt and pepper. Place one large slice of salmon on top, ribboning it to fit. Drizzle with a tablespoon of the sauce.
Garnish with some red onion, sprinkle with the dill fronds, salt and pepper, and serve with extra sauce on the side.
Whisk together the Dijon mustard, honey mustard, whole-grain mustard, ground mustard, sugar and vinegar in a medium bowl. Combine the olive and grapeseed oils in a small measuring cup. Slowly add the oil mixture to the mustard mixture, whisking constantly, until emulsified. Stir in the dill and salt.
You can make the sauce ahead and refrigerate it for up to a week.