While the bread is toasting, slice the bananas into 1/2-inch thick disks and put them into a bowl. Grate the lemon zest over the bananas. Cut the lemon in half and squeeze the juice into the bowl. Tear in the mint leaves and gently mix to combine.
Spread each piece of toast with 2 tablespoons of peanut butter and top each with half the bananas. Serve immediately.