Whisk the yolk with the garlic and hot pepper.
Slowly whisk in the oil, adding it drop by drop.
Whisk in the tomato paste or puréed red pepper, if you want a rust-coloured mayonnaise.
Season with salt, pepper, and lemon.
Spread on toasted or fried baguette croûtes.
Serve afloat on or alongside the fish stew.
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