Dice the onion and mince the garlic. Cook both down in a large pot over medium heat with 2 Tbsp of butter until the onions are soft and transparent (about 5 minutes).
Add the thawed lima beans and corn to the pot. Also add about a half teaspoon of salt, a generous amount of cracked pepper, and a half teaspoon of thyme. Saute the vegetables just until tender and warmed through (about 10 minutes).
While the lima beans and corn are sauteing, dice the tomato and roughly chop the parsley leaves.
Once the lima beans and corn are tender, add the tomato and parsley. Season with more salt, pepper, and thyme to taste.