Combine the garlic, thyme, black peppercorns and rapeseed oil in a small bowl. Using a sharp knife, score the pigeon breasts' skin lightly and rub in the marinade - leave for at least 20 minutes.
To make the gravy, combine the sugar and a few drops of water in a smallish, very clean saucepan and place over a high heat to caramelise. Once the sugar melts and has gone a dark golden brown, pour in the red wine. Add the thyme and the juniper berries. Cook the sauce until it is reduced by two thirds - this will take about 12-15 minutes. Season to taste, strain and keep warm.
Place the leeks in a saucepan of boiling, salted water and blanch for 10 seconds. With a pair of tongs, remove the leeks from the boiling water and plunge into a bowl of ice-cold water. When they have cooled, drain the leeks well.
Place the leeks in an ovenproof saucepan or baking sheet with a knob of butter. Transfer to the hot oven and cook until tender - this should take about 10-15 minutes.
Heat a griddle pan until hot. Season the pigeon breasts with salt and freshly ground pepper and place on the hot griddle pan. Cook for just a few minutes on each side until cooked to your liking, then remove from the heat and set aside in a warm place for at least five minutes.
For the mushrooms, heat a small frying pan. When hot, add the rapeseed oil and fry the garlic until golden brown. Add the sliced mushrooms and cook until softened. Be careful - the wood-ear mushrooms can pop and splutter quite ferociously.
Serve the pigeon with the leeks, mushrooms, and a drizzle of the red wine gravy.