Heat olive oil in a large pot over medium heat. Add chopped onions, sautéing until they become translucent and sweetly aromatic, about 5 minutes.

Stir in minced garlic, cooking for 1 minute until fragrant but not browned, releasing a warm, nutty aroma.

Add diced carrots and celery, stirring occasionally, until they begin to soften and their colors brighten, about 5 minutes.

Mix in rinsed red lentils, ensuring they are well coated with the oil and vegetables, which helps them cook evenly.

Pour in vegetable broth and diced tomatoes, stirring to combine. Add thyme and bay leaf, bringing the mixture to a boil.

Reduce heat to low, cover, and simmer for 20 minutes. The lentils should soften and the soup will thicken naturally.

Remove the bay leaf. Season with salt and pepper to taste, adjusting for personal preference.

Let the soup rest for 5 minutes off the heat to allow the flavors to meld before serving.