In a large skillet, heat oil over medium heat. Add onion and cook until soft (do not let it brown), about 8 minutes. Remove from heat and set aside.
Cut around base of each ear of corn and gently remove husks whole. Reserve husks. Brush silk off ears.
Grate kernels off corncobs into medium bowl, or slice off and pulse in processor to rough puree. Season with salt and pepper to taste. Mix in cooked onion, milk, and red pepper flakes. Chill mixture until firm.
Preheat oven to 350°F. To assemble 1 humita, choose a thinner, more flexible inner husk and tie the thin end into a knot. Split the husk up to the knot and set aside to use as tie. Lay 2 of widest husks side by side (narrow ends at top and bottom), overlapping a few inches to form rectangle. Spoon 1/4 cup filling into center, then fold Side Dish over to cover filling. Fold in top and bottom to make enclosed rectangular package. Tie crosswise with knotted husk. Repeat with remaining filling and husks.
Place finished humitas on baking sheet and bake until heated through, about 15 minutes.