Cook potato, peel & mash. In a bowl, combine yeast with lukewarm water. Allow to stand for about 3 minutes until foamy; add butter, salt, Italian seasoning, sour cream & potato & mix well.
Stir in flour, one cup at a time. When dough is completely blended, turn onto a lightly floured work surface. Knead dough about 10 minutes, until smooth & elastic.
Place dough in a greased bowl, cover with plastic wrap & allow to rise in a draft free place until doubled in bulk, about 1 hour.
Divide dough into 6 equal pieces & shape each piece into a round ball. On a lightly oiled surface, flatten each ball with a rolling pin to a 6-inch diameter. Spread each piece of dough with some of the remaining Boursin cheese.
Divide chicken/Boursin mixture between the 6 turnovers, laying it to one side leaving a half inch border. Fold each one in half to form turnovers.
Line a baking sheet with parchment paper. Place turnovers on baking sheet giving them space to rise a bit. Top each one with some caramelized onions & cheese. Cover turnovers with a tea towel & place in a draft free place to rise for about 15 minutes.
Preheat oven to 350 F.
Bake turnovers until a nice golden brown color. Remove from oven & allow to cool on a wire rack.