Combine the tuna, onion, parsley, capers, harissa, salt, and pepper. Keep the mixture loose rather than compacting it.
Warm the oil in a wide skillet over medium heat until a small pastry scrap sizzles gently.
Fill and Fry:
Place one pastry sheet on a plate. Spoon one-eighth of the tuna mixture on one half, make a shallow well, and crack an egg into it. Fold the pastry over into a half-moon.
Slide the brik carefully into the oil and fry for 60 to 90 seconds per side until patchy golden and crisp while the yolk remains soft. Drain briefly on paper towels.
Repeat with the remaining sheets, filling each only when the oil is ready so the pastry does not soften.
Serve:
Serve the brik immediately with lemon wedges and additional harissa at the table.
Notes
Malsouka dries quickly, so keep unused sheets covered with a barely damp towel.
For a firmer yolk, fry each side about 30 seconds longer over slightly lower heat.