Slice the tomato to your desired thickness and toast the Rye Bread. Assemble the sandwich by placing the tomato slices down first, then onion, then snap the lettuce in half and put that on, and finally the top piece of vegan bread. Not very complicated at all and very, very good!
When I make it, I usually slice the ingredients a half an hour ahead and place it in the fridge, it makes the onion sweeter and the tomatoes better. Also the contrast of hot toasted vegan bread to cold veggies is delightful.