Preheat oven to 375F (190C).

Lightly oil a 9x13-inch baking dish and set aside.

In a bowl, combine minced garlic, chopped fresh basil, chervil (or parsley), and marjoram (or dried marjoram).

Layer sliced red onion, potatoes, zucchini, portobello mushrooms, and tomatoes in the prepared baking dish in slightly overlapping rows.

Drizzle each layer with olive oil and sprinkle with the garlic-herb mixture, salt, and pepper.

Repeat layering until all vegetables are used.

Tamp down the vegetables and drizzle with any remaining olive oil.

Cover the dish tightly with aluminum foil.

Bake for about 50 minutes, or until the vegetables are tender.

Uncover the dish and bake for an additional 10 to 15 minutes to allow the vegetables to brown slightly.

Remove from oven and let stand for about 30 minutes before serving to allow flavors to meld.

Pour off the accumulated juices from the pan into a saucepan.

Bring the juices to a boil over high heat.

Reduce the heat to medium and cook until the juices are reduced to a syrupy consistency, approximately 20 minutes.

Spoon the reduced sauce over the vegetables.

Cut the vegetable medley into squares and serve at room temperature.