Clean and wash the aubergine and cut lengthwise into 12 thin slices. Place them side by side on the work surface and sprinkle with salt. Leave to stand for about 30 minutes.
Meanwhile peel and chop the onions and garlic. Heat 1 tsp. oil in a pot. Fry half of the onions and garlic in it. Add frozen spinach and 100 ml water. Steam over medium heat for about 15 minutes. Season to taste with salt, pepper and nutmeg. Drain and let cool down a little.
Wash the rosemary and dab dry, pluck and chop the needles. Heat 1 tsp. oil in a pot. Sauté the rosemary with the rest of the onions and garlic. Add tomatoes, bring to the boil and simmer for 5-6 minutes. Season with salt, pepper and sugar.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Crumble the feta, mix with the spinach. Dab aubergines dry. Spread the spinach on top and roll up. Place the rolls in an ovenproof dish, pour tomato sauce over them, grate cheese over them. Bake in a hot oven for about 45 minutes.
Meanwhile cook the rice in 400 ml boiling salted water according to the instructions on the packet. Serve with the cannelloni.