Place a large heavy-bottomed pot over medium-high heat, or turn on your Instant Pot sauté function normal.
Add the oil and heat until shimmering, then add the diced onion and sauté until translucent, 2-3 minutes.
Add the garlic and sauté 2 minutes more.
Add the shredded zucchini and carrots and sauté about 5 minutes, until the vegetables are soft.
Add the vegetable broth, red lentils, tomatoes, and thyme.
If you are using the Instant Pot, give the bottom a very good stir with a wooden spoon to ensure no lentils are sticking. This will prevent the burn warning from going off. Set the Instant pot to 10 minutes at manual high pressure. When the cooking cycle is done you can use either a quick release or natural release.
If you are cooking on the stovetop, reduce the heat to medium-low an simmer with the lid ajar until the lentils are soft - about 20 minutes.
Serve hot over rice or pasta.
