In a resealable container, place the sunflower seeds and cover with water. Soak for 24 hours at room temperature. (They should double in size.) Drain the water, add more water to cover, and then drain again.
Place 1/2 of the sunflower seeds, lemon juice, and mustard in a food processor. Pulse until finely chopped, about 10 times.
Add the remaining sunflower seeds, parsley, paprika, celery, scallions, and salt and pepper. Pulse an additional 10 times, or until texture resembles tuna salad.
Layer sourdough bread (optional to toast) with spinach, sliced Roma tomato, and sunflower mixture. Optional to sprinkle hemp seeds on top for some additional healthy fats and to add some crunch.