Make the marinade by whisking the brown sugar into the tomato puree in a bowl until dissolved. Stir in the pepper flakes, vinegar
, garlic and thyme. Wash the collars under cool tap water and dry off with paper towels. Place the collars in a baking dish
and cover with the marinade
. Refrigerate overnight.
Preheat the oven to 350 degrees F.
Slice the apple and fennel thinly on a mandolin, and then place the slices in water for a couple of minutes to preserve freshness.
Place the almonds on a small baking sheet and bake for 10 minutes. Let cool, then chop.
Heat a medium saute pan with a little bit of oil and sear each hamachi collar until golden brown. Transfer to a baking sheet and finish cooking in the oven, 10 to 12 minutes.
Carefully mix the apple, fennel, grapefruit, lemon and oranges in a bowl with the olive oil and chives. Add salt and black pepper to taste.
To serve, place the salad in the center of 6 plates. Put 1 collar skin-side down on top and garnish with the toasted almonds.