Start by chopping the carrots into thick, uniform pieces and dicing the onion and celery for even cooking.
Add the chopped carrots, celery, and diced onion into the bowl of your large slow cooker, spreading them out evenly.
Pour in the canned diced tomatoes along with their juices, distributing them over the vegetables.
Pour the vegetable broth over everything, ensuring the ingredients are submerged but not overloaded—about 4 cups should do.
Place the sprigs of thyme and bay leaf on top, then sprinkle with salt and pepper to taste.
Cover the slow cooker with its lid and set it to low. Let it simmer for 6 to 8 hours until the vegetables are tender and the flavors meld beautifully.
Once cooking is complete, remove the thyme sprigs and bay leaf, then give the soup a gentle stir with a wooden spoon.
Taste the broth and adjust seasoning with more salt and pepper if needed, or add a squeeze of lemon for brightness.
Ladle the hot soup into bowls, revealing a colorful, hearty mixture of tender vegetables and a rich, aromatic broth.
Finish with a drizzle of olive oil or a sprinkle of fresh herbs if desired, and serve with crusty bread for a complete, satisfying meal.
