Begin by chopping the cabbage into roughly 1-inch pieces, removing any tough outer leaves. Dice the onion and peel and slice the carrots into small rounds. Mince the garlic finely.
Add the chopped cabbage, sliced carrots, diced onion, and minced garlic into the crockpot. Pour in the can of diced tomatoes with their juice, and then add the vegetable broth.
Sprinkle in the dried thyme, tuck in the bay leaf, and season generously with salt and pepper. Give everything a good stir to combine the flavors evenly.
Cover the crockpot and set it on low for 6 to 8 hours, or on high for 3 to 4 hours. As it cooks, the vegetables will soften and the broth will turn fragrant and flavorful.
Check the soup after the cooking time; the cabbage should be tender and slightly translucent, and the broth flavorful and fragrant. Remove the bay leaf.
Taste the soup and adjust the seasoning with more salt and pepper if needed. For a bright note, stir in a teaspoon of vinegar or lemon juice.
Give the soup a gentle stir, ladle it into bowls, and serve hot. Enjoy the comforting warmth and tender vegetables in every spoonful.