Peel or scrub vegetables according to your minhag. Slice zucchini and yellow squash diagonally into 1/4" slices. Slice eggplant lengthwise into 1/2" slices. Stack the slices and cut into sticks wide. Core peppers and cut them into strips approximately 1" wide. Slice the onion into rings 1/4" thick.
Toss vegetables with oil, salt, and pepper in a large bowl.
Heat your grill pan over medium-high heat until a drop of water sprinkled on it sizzles and disappears. Lower flame to medium and spread the vegetables in the pan. Cook 7 to 8 Minutes or until black grill marks appear on the vegetables tossing once during cooking.
4 servings