Add the olive oil to a large cast-iron skillet; heat over medium-high heat until hot. Add the smelt and crush into a paste using the back of a wooden spoon. Add the garlic, olives and capers, and cook until the garlic begins to color around the edges. Add the pepper flakes and stir to combine. Add the tomatoes and cook until they begin to break down, about 10 minutes. Remove from the heat.
Set a cast-iron grill pan over medium-high flame; heat until hot. Gently score the mackerel on both sides, deep enough to pierce into the fillets, but not to the bone. Brush with olive oil and sprinkle with salt and pepper.
Grill the fish until charred, cooked through, and the eyes have whitened, 5 to 6 minutes per side.
Serve the mackerel over a bed of the tomato sauce. Garnish with parsley.