Preheat the oven to 375˚F. Bake the potatoes in their skins for 1-1¼ hours until tender, then remove the flesh and mash until smooth.
Mix the ricotta cheese with a pinch of salt and white pepper and then add the flour along with the mashed baked potatoes. Make a well in the middle, add the beaten egg and combine the mixture with floured hands. Work the dough until a smooth dough has formed; about 5 minutes.
Cut the dough in half and shape each piece into a long cigar, about 1/2" thick. Using the back of a floured butter knife, cut each length into 1" pieces to make the gnocchi. Gently make a dent in each one – it will allow the gnocchi to hold more sauce.
Add the gnocchi to a large pan of boiling water, stirring briefly to stop them sticking together. Simmer for 1½-2 minutes, until they start to float. Drain and leave the gnocchi to dry for 1-2 minutes.
Heat a frying pan over medium-high heat and add a little olive oil. Add the gnocchi to the pan with a pinch of salt and black pepper and sauté for 1-2 minutes on each side until colored. Add the peas, butter and sage. Toss, then add the lemon zest.