Build depth of flavor in meatless dishes in layers: sauté the vegetables and onion until browned, add umami sources such as tomato paste, dried mushrooms, or aged cheese, use homemade vegetable stock instead of water, and finish the dish with acid and fat. Every part of the richness meat adds to the pot can be supplied by another ingredient; often, one of these layers is what is missing.
The first layer is browning. In a meat dish, browning the meat leaves deep flavor on the bottom of the pot; in a meatless dish, vegetables do that job. Sauté the onion patiently until it passes pale gold and begins to turn brown. Let mushrooms cook in the pan until they release their liquid, the liquid evaporates, and they brown. Roast eggplant, zucchini, and bell pepper in the oven or on the grill until their edges are charred. Deglaze the browned bits left on the bottom of the pot with a spoonful of water and add them to the dish; much of the flavor is there.
The second layer is umami. Cooked-down tomato paste is the strongest and easiest source of umami in a meatless dish. Soak dried mushrooms in warm water, chop them, and add them to the dish; strain the soaking liquid and use it as well. Sun-dried tomatoes, grated aged cheese, and a few drops of soy sauce do the same job. Using two or three of these sources together noticeably strengthens the effect. Since these ingredients also add salt, taste the dish and adjust its salt after adding them.
The third layer is liquid. A vegetable dish cooked in water has only the flavor of the vegetables themselves; homemade vegetable stock gives it a foundation from the start. Put onion, carrot, celery, the green part of a leek, mushroom stems, and a few parsley stems on the stove in cold water; simmer for 45 minutes, then strain. If you roast the vegetables first, the stock will be darker and deeper. If using bouillon, account for its salt and taste before adding any extra salt to the dish.
The fourth layer is richness. The fat and gelatin in meat leave a full sensation in the mouth; in a meatless dish, fat, tahini, walnuts, and yogurt provide it. A knob of butter or a spoonful of olive oil added at the end of cooking extends the flavor. The satisfying flavor of muhammara and walnut sauces comes from the fat in the walnuts. A vegetable dish served with tahini sauce or strained yogurt feels much more filling than it does on its own. Cumin, isot pepper, and toasted red pepper flakes also add a warm, deep note to the base.
The final layer is brightness. A dish built on deep, roasted flavors can feel heavy; a few drops of lemon juice or a spoonful of pomegranate molasses squeezed in as you turn off the heat lifts that heaviness. Fresh parsley, dill, or mint sprinkled on at serving time does the same. You do not need every layer in every recipe; if a meatless dish tastes flat, taste for the missing layer and add only that one. Often, what is missing is patient browning.