Put the potatoes in a pan of cold water, bring to the boil and cook for 10-15 minutes until tender. Drain, mash until smooth and set aside to cool.
Meanwhile, put the peas in a small pan of boiling water and cook for 4-5 minutes until tender. Drain, transfer to a food process or blender and purée until smooth.
Finely chop the mint and put in a large bowl with the flour, mash and pea purée. Mix well and season with freshly ground black pepper. Divide the mixture into 2 or 3 pieces and roll on a lightly floured surface into sausages about 2.5cm in diameter. Cut into 3cm pieces, pressing each one lightly with the back of a fork.
Bring a large shallow pan of water to the boil. Drop the gnocchi in (this can be done in batches) and cook for 2-3 minutes or until they rise to the surface. Remove from the pan with a slotted spoon and divide between serving plates.
In a small frying pan, melt the butter with the oil, add the sage leaves and cook for 1-2 minutes until the sage is crisp. Pour over the gnocchi and serve topped with the pea shoots.