Dry the cubed whole-grain bread overnight in a large bowl until it has the texture of day-old bread.
Cook the green lentils in 1 1/2 cups of water until tender, about 20 to 30 minutes, then set aside.
Preheat the oven to 350 degrees Fahrenheit and prepare a 9×13-inch baking dish with nonstick spray. Prepare a flax egg by combining flaxseed meal with water and set aside.
Sauté onions and celery in olive oil with a pinch of salt and pepper until fragrant and translucent, about 5 minutes, then set aside.
Combine the dried bread cubes with most of the vegetable broth, cooked vegetables, flax egg, cooked lentils, and dried sage. Adjust the consistency as needed to be moist but not soggy.
Transfer the mixture to the prepared baking dish, cover with foil, and bake for 45 minutes. Then uncover and bake at 400 degrees Fahrenheit until the top is browned and crisp, about 10 to 15 minutes.
