Heat olive oil in a large pot over medium heat. Add the diced onion and cook for about 5 minutes until soft and translucent.
Add the minced garlic, cumin, and turmeric. Stir constantly for 1 minute. Toasting these in oil "blooms" the flavor, making the stew taste significantly more complex.
Add the rinsed red lentils, the canned tomatoes (with juices), and the broth. Stir to scrape up any flavorful bits from the bottom of the pot.
Bring the mixture to a boil, then immediately reduce the heat to low. Cover and simmer for 15–20 minutes. You’ll know it’s done when the lentils have softened and the stew has thickened.