Fried shiitake mushrooms with garlic, a splash of soy sauce, and lemon juice. I usually use garlic powder for convenience.

If you're ok with lacto-ovo vegetarian dishes, it goes good with a fried egg and rice as a quick and tasty meal. Sometimes I'll have some kimchi with it too if I'm in the mood and have some on hand. (Note: a lot of kimchi is not vegetarian -- it often includes fish/seafood ingredients. The store near me sells both a vegetarian and a non-vegetarian version though and despite not being a vegetarian I prefer the taste of the vegetarian one slightly more, so I usually get that if they have it. Check the ingredients/labels if you're unsure.)

The mushrooms are also a fantastic accompaniment to steak when you're doing non-vegetarian cooking. Fry them in the same pan after making a steak on the stove and you can get something that can, frankly, eclipse the steak...

Another vegetarian dish I make sometimes (particularly in the summer) is basically just a chickpea salad. I usually use canned chickpeas for that with raw sliced vegetables like tomato, cucumber, red bell pepper, etc. I add salt, lemon juice, and olive oil and eat it with some toasted pita bread. Sometimes I will add avocado, chopped shallots, chunks of cheese, and/or a hard boiled egg if I have them handy and am in the mood -- but it's pretty good even without any of those extras. With canned chickpeas, I've found that I don't like the version with firming agents added (usually calcium chloride, IIRC), so I always check the labels at the store and get ones without it.

A third vegetarian dish I like is a variant on spaghetti puttanesca. While I boil pasta, I prepare a sauce in a pan using canned tomatoes, olive oil, capers, kalamata-style black olives, salt, a pinch of msg, and cayenne pepper. Finish the pasta in the sauce. One of the things I really like about this dish is that all the ingredients are shelf-stable so you can keep them on hand and just make it whenever. (My version notably leaves out the anchovies from more common variants of the dish.)