In a small bowl, mix warm milk, 1 tbsp sugar, and yeast. Let sit for 5–10 minutes until foamy. In a large bowl, whisk flour, remaining sugar, and salt. Add yeast mixture, egg, and butter to the flour mix. Combine into a sticky dough.

Knead on a floured surface or in a mixer with a dough hook for 8–10 minutes until smooth and elastic. Place dough in a greased bowl, cover, and let rise for 1 hour or until doubled in size. In a bowl, combine drained tuna, mayo, onion, calamansi/lemon juice, and season to taste. Keep chilled until ready to use. Punch down risen dough. Divide into 16 equal pieces.

Flatten each piece into a small disc and place about 1 tablespoon of the tuna filling in the center. Pinch the edges to seal the filling inside and shape into a ball or slightly oval roll. Roll each piece lightly in breadcrumbs and place on a baking tray lined with parchment.

Cover the tray loosely with a towel and let the rolls rise again for 30–45 minutes. Bake in a preheated oven at 350°F (175°C) for 18–20 minutes, or until golden brown. Cool slightly before serving. Best enjoyed warm!