Make the vinaigrette first. Place the cubed beet, vinegar, shallot, and a pinch of salt in a blender or food processor. Blend until smooth, then slowly drizzle in the olive oil while the machine is running. Add honey or maple syrup and blend again to combine. Set aside — this can be made up to 2 days ahead.
Press the smoked fish dry. This is important. Line a small plate with paper towels, break the mackerel into it, and press gently with another towel on top for about 5 minutes. You want most of that olive oil gone but don't squeeze so hard you lose all the meat's texture.
Mix the fritter batter. In a medium bowl, mash the drained fish with a fork until no large pieces remain (you want it mostly broken down). Add the egg and stir to combine. Fold in the flour, panko, red onion, Dijon, lemon zest, smoked paprika, salt, and pepper until well mixed. If your mixture feels too wet to form patties, add another tablespoon of panko.
Form the patties. Scoop about 2 tablespoons of mixture per fritter (a small cookie scoop works perfectly here) and shape into flat patties about 1/3 inch thick. Place on a plate and refrigerate for at least 10 minutes — this helps them hold together in the oil.
Heat your oil. In a large skillet over medium-high heat, add enough oil to come about 1/2 inch up the sides (you should have about 375°F when you test with a drop of batter). The oil is ready when a small piece of batter sizzles immediately and rises to the surface.
Fry in batches. Working in batches to avoid crowding, cook fritters for about 2 minutes per side until golden brown and crisp. Drain on paper towels. The total cooking time is roughly 4-5 minutes, so this moves quickly once your oil is hot.
Serve immediately. Plate the fritters with a generous spoonful of the beet vinaigrette over or alongside them. Garnish with extra cracked pepper and a few torn leaves of watercress if you have it.
