Cook the lentils, garlic and celery in a pot of water until tender. This should take about 25 minutes. Drain any excess water from the lentils and set them aside. Discard the celery.
Meanwhile slice the onion and cut the cherry tomatoes in half.
Heat the olive oil in a large frying pan over medium. Add the onion and cook until softened, for about 5 minutes.
Add the spinach and cook for 2 minutes, until just wilted.
Add the tomatoes, lentils & herbs and stir well. Cook for a couple of minutes until warmed through, then stir in the lemon juice.
Season with salt and pepper.