Heat the oven to 200C/fan 180C/gas 6. Rinse the lentils well, then simmer in boiling water for 15 minutes until starting to go soft. Fry the onion in a spray of oil until soft but not brown, and allow to cool. Drain the lentils well and put in a blender with the cooled onion, mushrooms, oats and lemon zest. Pulse until the mix is combined (but don’t blend to a mush). Season, and roll into 16 balls, then put them on a lined baking sheet. Bake for 30 minutes until golden and firm, turning them halfway through.