Thaw spinach and squeeze out as much water as possible. Sauté onion in 2 tablespoons butter until clear. Add scallions and cook until they wilt. Add spinach and seasonings. Toss lightly. Remove from heat and add milk. Beat eggs lightly in another bowl and add feta cheese, crumbled. Add to spinach mixture; mix well. With pastry brush, coat bottom and sides of 9 x 13-inch pan with melted butter. Line with 8 sheets of phyllo (found in frozen bread section of supermarket), brushing each set with melted butter. Do not trim overhang. Pour in spinach mixture and fold overhanging sections of phyllo sheets back over filling. Top with 8 or more sheets of buttered phyllo dough. Trim overlap. Bake at 350° for 45 minutes.