Grandma's Old-Time Squash Casserole

GRANDMA’S OLD-TIME SQUASH CASSEROLE6 T. butter, divided3 lbs. yellow squash, sliced ¼” thick2 c. chopped onionkosher salt2 eggs, beaten1 (8-oz.) container sour cream1 c. shredded sharp cheddar cheese½ c. shredded Swiss cheese½ c. Miracle Whip or mayo1 T. chopped fresh…