Heat oil in a large skillet over medium-high heat. Saute onions until softened and just beginning to brown, about 8 minutes, stirring often. Add tempeh and cook until golden brown. Season with salt and pepper to taste.
Spray a 9 x 11 baking dish with cooking spray or oil and layer potatoes on the bottom, overlapping. Season with salt and pepper to taste.
Cover potatoes with tempeh mixture. Top tempeh layer with baked beans and top baked beans with tomato soup.