Peel the small red onion and mince it finely. Rinse the two large red peppers and cut them in half lengthwise. Remove the stems, the white membranes, and the loose seeds from the inside, leaving clean, hollow shells.
Open the two tins of tuna and drain the water completely. Transfer the drained tuna to a bowl and break the pieces apart with a fork. Add the minced red onion, the tablespoon of sugar-free mayonnaise, and a pinch of turmeric. Mix everything together thoroughly until you have an even tuna salad.
Place the four pepper halves with the open side facing up on a work surface or cutting board. Divide the tuna salad evenly with a spoon over the hollow spaces of the peppers. Press the filling gently so the mixture stays firmly in the vegetable.
Finish by sprinkling a little Celtic sea salt over the filling for extra flavor. Serve the stuffed peppers immediately as a lunch, or cover and refrigerate until ready to eat.