Lightly butter the bread, it acts as a seal, then apply mayo or mustard. Store in the fridge overnight.
My mother did this my entire childhood and it drove me bananas. Butter in peanut butter sandwiches, butter in bologna sandwiches, butter in tuna sandwiches .... I love my mother but I wished wished wished I could have normal sandwiches with normal condiments. She always said, "you don't want the bread to dry out, do you?"
I think it was 5th or 6th grade before it occurred to me to make my own. Somehow, she never challenged me to do it earlier.