In the microwave or in a small saucepan, warm the milk until it feels just slightly warm to the touch., Combine the warm milk with the eggs, sourdough starter and salt, then mix in the flour. Don't over-mix; a few small lumps are OK. The batter should be thinner than a pancake batter, about the consistency of heavy cream., Heat a muffin or popover pan in the oven while it's preheating to 450°F., Carefully remove the hot pan from the oven, and spray it thoroughly with non-stick pan spray, or brush it generously with oil or melted butter. Quickly pour the batter into the cups, filling them almost to the top. If you're using a muffin tin, fill cups all the way to the top. Space the popovers around so there are empty cups among the full ones; this leaves more room for expansion., Bake the popovers for 15 minutes, then reduce the oven heat to 375°F and bake for an additional 15 to 20 minutes, until popovers are golden brown., Remove the popovers from the oven and serve immediately.