Cook the beans. Add the soaked beans into a medium pot, cover with water by several inches, and bring to a boil over high heat. Reduce the heat to medium-low, season with salt, and simmer, uncovered, until the beans are fully tender, 45-90 minutes (depending on the size/freshness of your beans). Top with water as necessary to keep the beans submerged at all times. Sample a bean at the 40-minute mark to see how tender the beans are. You’re looking for a tender bean with a tiny bit of bite. Once cooked, drain the beans.
Sauté the vegetables. Heat the olive oil in a medium skillet over medium heat. Add the onion and garlic and sauté until translucent, 5-7 minutes. Add the celery, bell pepper and soy sauce, and sauté until the vegetables begin to soften, 3-5 minutes.
Preheat the oven. Arrange a rack in the middle of the oven. Heat the oven to 375°F/190°C.
Process all the bean loaf ingredients. Add ½ cup rolled oats into the food processor fitted with an S blade and process until finely ground. Add the sunflower seeds and pulse until the seeds are coarsely ground. Add the rest of the ingredients (including the remaining rolled oats) and pulse to combine, pausing to scrape down the sides as necessary. Be careful not to purée.
Season. Taste and adjust the flavor as needed.
Bake. Transfer the bean mixture into a parchment-lined loaf pan, spread, and press into an even layer. Brush the top with a little bit of oil to help the loaf retain moisture. Bake the lentil loaf, uncovered, until golden brown on the edges and slightly dry to the touch, 35-45 minutes. Remove from the oven and let cool slightly, 10-15 minutes. Then gently remove from the pan.
Serve. Carefully cut the loaf into 12 even slices. Serve with a side of mashed potatoes and steamed vegetables.
Store. Leftover bean loaf keeps well in an airtight container in the refrigerator for up to 5 days. For longer term storage, freeze sliced bean loaf in an air-tight container, separating each slice with a piece of parchment paper, and freeze for up to 3 months.
Reheat. Slice the loaf first (if it’s not sliced already). Then lay the individual slices on a bare baking sheet and reheat in a 350°F/175°C oven until warmed through (the time will vary depending on whether reheating from refrigerated or frozen).