In a 5- to 6-quart pan over medium heat, combine 1 tablespoon butter, onions, caraway, and 1/2 cup water. Cover and stir occasionally until onions are very limp, 10 to 15 minutes (add a few more tablespoons water if liquid evaporates before onions are ready), then uncover and stir often until liquid evaporates. Remove from heat.
Squeeze excess liquid from spinach. Mix spinach with onions. Add 2 1/2 cups of the cheese and salt to taste; mix well.
Lay crêpes slightly apart on a counter, pale side up. Spoon equal amounts of spinach filling down center of crêpes; roll to enclose.
Set filled crêpes, seams down and side by side, in a buttered shallow 3-quart casserole (9 by 13 in.).
Bake in a 375° regular oven or 350° convection oven until hot in center, 15 to 20 minutes. Sprinkle remaining cheese in a band down center of crêpes. Return to oven and bake until cheese melts, about 3 minutes. With a wide spatula, transfer crêpes to plates. Add salt to taste.