In a saucepan put lentils in the water. Cover and bring to a boil over high heat. Reduce to simmer. Cook for 25-30 minutes or until tender.
Heat 1 tbsp oil over medium heat in a large skillet. Add onion and celery, cook for 7-8 minutes or until tender, stirring frequently. Add peppers, garlic, and half the salt and black pepper. Cook for another 5-6 minutes until peppers are slightly tender, stirring frequently.
Drain lentils and transfer to a bowl. Stir in vinegar, dill, oregano, lemon juice, sauteed vegetables, remaining salt and pepper, and remaining oil.
Divide the spinach between the plates and top with lentil salad and feta cheese.