Heat the olive oil in a large saucepan over medium heat. Add the chopped onion and cook for 5 to 6 minutes, stirring occasionally, until soft and translucent.

Stir in the minced garlic and ground cumin, cooking for about 1 minute until fragrant. Make sure to stir constantly to prevent the garlic from burning.

Add the rinsed brown lentils and vegetable broth to the saucepan, scraping up any browned bits from the bottom. Bring the mixture to a boil, then reduce the heat to low, cover with a lid, and simmer for 20 to 22 minutes until the lentils are tender.

While the lentils are simmering, whisk the tahini, fresh lemon juice, and 2 tablespoons of warm water together in a small bowl until a smooth, pourable dressing forms.

Remove the saucepan from the heat and stir in the baby spinach until it is fully wilted. Gently fold in the lemon-tahini mixture, then season with salt and black pepper to taste.

Garnish the warm lentils with a sprinkle of fresh parsley or red pepper flakes, and serve alongside toasted pita bread or over warm brown rice.