Position an oven rack in the center of the oven and preheat to 400 degrees F. Spray four 10-ounce ramekins with vegetable oil cooking spray.
Heat the oil over medium-high heat in a medium skillet. Add the pancetta and cook until browned, about 6 minutes. Remove the pancetta using a slotted spoon and drain on paper towels. Add the shallots, salt and pepper in the same skillet. Cook over medium-high heat until soft, about 3 minutes.
In a medium bowl, mix together the cooked pancetta, shallots, bread, eggs, spinach, gruyere and milk until all the ingredients are moistened.
Spoon the mixture into the ramekins. Place the ramekins on a small baking sheet and bake until golden brown, about 25 minutes.
Cool the ramekins for 20 minutes. Using a paring knife, loosen the edges of the ramekins and unmold onto serving plates. Serve warm or at room temperature.