Activate the yeast by combining warm milk with sugar and active dry yeast in a mixing bowl. Let sit for about 10 minutes until frothy.

Make the dough by adding melted butter and salt to the yeast mixture, then mix in flour until a shaggy dough forms. Knead on a floured surface for 5-7 minutes until smooth. Let rise in a greased bowl covered with a towel for 1 hour.

Caramelize the onions by heating olive oil and butter in a skillet over medium heat. Add sliced onions and cook for 25-30 minutes until golden brown, adding thyme, black pepper, and a pinch of salt and sugar at the end.

Prepare the filling by rolling out the risen dough into a rectangle, spreading caramelized onions on top, and sprinkling Gruyère cheese evenly.

Shape the wreath by rolling the dough tightly into a log, slicing in half lengthwise, and twisting the halves to form a wreath. Pinch the ends together.

Final rise by brushing the wreath with melted butter and sprinkling with rosemary and flaky salt. Let rise for another 20 minutes.

Bake at 375°F for 25-30 minutes until golden brown and hollow sounding when tapped.

Serve warm by brushing with melted butter again, allowing to cool slightly before tearing apart.