Preheat oven to 180°C. Heat oil in a non-stick frypan over medium heat. Add onion and bacon and cook, stirring occasionally, for 8-10 minutes until bacon fat has rendered and onion has softened. Stir in garlic. Cook for 2 minutes or until softened. Stir in rice and grated zucchini. Reduce heat to low.
Add 1 ½ cups (375ml) warm water, ½ cup at a time, stirring constantly for 4-5 minutes, until about two-thirds of the water has been absorbed each time. Add another ½ cup warm water and bring to a simmer. Remove from the heat and season. Stir in lemon zest, mascarpone and half the Gruyere.
Spoon into a gratin dish (the rice should be like a loose risotto) and scatter over zucchini rounds and remaining Gruyere. Cover with baking paper, then foil and bake for 25-30 minutes until rice is tender. Remove foil and baking paper and bake for a further 10 minutes or until cheese is deep golden and bubbling. Scatter with dill sprigs and serve with lemon wedges.
