Heat the olive oil in a large skillet over medium heat. Add the diced onion and minced garlic, then cook for 2 minutes until the onion softens.
Stir in the sliced mushrooms and fresh thyme leaves. Cook for 4 minutes, stirring now and then, until the mushrooms release their liquid and start to brown.
Add the drained French lentils, vegetable broth, salt, and black pepper to the skillet. Stir to combine.
Bring the mixture to a gentle simmer and cook for 6 minutes, stirring once or twice, until the lentils are heated through and the broth thickens slightly.
Remove the skillet from the heat and stir in the chopped fresh parsley before serving.