A pastry pocket of warm Boursin cheese and mushrooms with Swiss chard and seasoning.
1. Position rack in lower third of oven, then preheat to 425F. Line a baking sheet with parchment. Gently lift pie shells out of foil plates and place on prepared sheet. Refrigerate until ready to use.
2. Melt butter in a large frying pan over medium-high. Add shallots and cook until softened, 2 to 3 min. Add mushrooms and chard stems. Season with pepper. Cook until mushrooms begin to brown, 6 to 8 min. Stir in chard leaves and crumble in 1/4 cup cheese spread.
3. Remove pie shells from refrigerator and let sit at room temperature for 2 to 3 min. (This will allow the edges to soften and avoid cracking when folding over.) Divide mushroom mixture between pie shells, leaving a 1 1/2-in. border. Sprinkle remaining cheese over top, then lift and fold pastry edges over part of filling. (Centre of pie should not be covered with pastry, and edges will be uneven). Lightly brush pastry tops with milk.
4. Bake, rotating sheet halfway through, until crusts are golden and cooked through, 15 to 20 min. Let stand 5 min, then cut into wedges and divide among 4 plates.