Cook the chickpeas. Add the soaked chickpeas 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 chickpeas are fully tender, 45-90 minutes (depending on the size/freshness of your chickpeas). Top with water as necessary to keep the chickpeas submerged at all times. Sample a chickpeas at the 40-minute mark to see how tender the chickpeas are. You’re looking for a tender chickpea with a tiny bit of bite. Once cooked, drain the chickpeas.
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 carrot, celery, 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 chickpea loaf ingredients. Add the walnuts into the food processor fitted with an S blade and process until coarsely ground. Add the rest of the ingredients 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, adding more salt for saltiness, thyme for an earthy flavor with lemony-minty undertones, rosemary for a woodsy flavor with aromas of lemon and pine, and sage for a minty flavor with lemony undertones.
Bake. Transfer the chickpea 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 chickpea loaf, uncovered, until golden brown on the edges and slightly dry to the touch, 40-45 minutes.
Cool. 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 chickpea loaf keeps well in an airtight container in the refrigerator for up to 5 days. For longer term storage, freeze sliced chickpea loaf in an air-tight container, separating each slice with a piece of parchment paper, 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).