Heat the olive oil in a large pot over medium heat. Add the diced onion and cook for 4 minutes until it softens. Stir in the minced garlic and cook for 1 minute more.
Add the cumin, smoked paprika, coriander, salt, and black pepper. Stir for 30 seconds to toast the spices.
Pour in the red lentils, crushed tomatoes, and vegetable broth. Bring the mixture to a boil, then reduce the heat to low. Simmer uncovered for 20 minutes, stirring occasionally, until the lentils break down and the stew thickens.
Taste and adjust salt or pepper if needed. Ladle the stew into bowls and top each serving with a spoonful of plain yogurt and a sprinkle of za’atar.