3 Tbs. fresh lemon juice
1 Tbs. plus 2 tsp. chopped fresh marjoram
1 tsp. finely grated lemon peel
¼ cup plus 3 Tbs. extra-virgin olive oil
½ lb. potatoes, unpeeled, cut in 1-inch cubes
1 lb. summer squash cut on diagonal into 1-inch slices
¼ cup minced red onion