Special equipment: a nonstick 24-cup mini muffin tin and a 2 1/2-inch round cutter
Heat the oil in a medium skillet over medium-low heat until it shimmers. Add the onion and a pinch of salt and cook, stirring occasionally, until very soft and golden, about 15 minutes. Set aside.
Preheat the oven to 375 degrees F. Spray a nonstick 24-cup mini muffin tin with cooking spray.
Meanwhile, unroll the pie crusts and use a 2 1/2-inch round cutter to cut out 24 rounds (12 rounds from each crust). Fit the rounds in the cups of the prepared muffin tin, pushing the bottoms down and pleating the sides slightly.
Whisk the half-and-half, egg and egg yolk, cayenne and some salt and pepper together in a medium bowl until very well combined. Stir in the chives and transfer to a liquid measuring cup. Divide the onions and ham evenly among the 24 dough-lined muffin cups. Divide the half-and-half mixture evenly among the cups.
Bake until the quiches are puffed and golden, about 20 minutes. Let the quiches cool in the muffin tin about 5 minutes, then remove to a wire rack. Serve slightly warm or at room temperature.