Cut the onions in half, peel them and then cut them into 5-6mm (¼") thick half-moon shapes.
Heat the oil in a 28cm or 11" skillet or frying pan over a medium heat, add the onions and cook for 30-40 minutes or until nicely nice and golden brown. Stir occasionally, and if the onions start to colour too quickly, turn the heat down a little.
Peel the garlic cloves and chop them as finely as you can.
Pick the sage leaves from the stems and then shred them as finely as you can.
When the onions are nicely golden, move them to a bowl for a few minutes.
Return the pan to a high heat, and when it is hot, throw in the minced chicken and cook until it is all coloured up. This will take around 5 minutes. This process should leave a wonderful nutty brown "fond" on the base of the pan. If this starts to go black or smells burnt, add a splash of water and turn down the heat and stir constantly.
Add the garlic, half of the shredded sage, salt and pepper, then cook for another 60-90 seconds, stirring constantly.
Stir in the caramelised onions and pour in the chicken stock, then stir to remove all of the caramelised bits from the base of the pan, reduce the heat to low, add a lid and cook for as long as it takes to cook your pasta.
Cook the pasta in a 20cm or 8" saucepan in well-salted water (I use 1½ teaspoons of salt in a pan this size) until it is al dente.Then drain and reserve around 75ml or ⅓ Cup of the pasta cooking water.
Whilst the pasta is cooking, melt the butter in a small 20cm or 8" frying pan over a medium-high heat.When the butter begins to foam, throw in the remaining sage and cook for 60 seconds, then add the breadcrumbs and cook until golden. When they are golden, transfer them to kitchen paper to drain any excess oil.
Add the drained pasta to the minced chicken and mix to combine, adding as much of the pasta cooking water as needed to form a nice, silky coating on the pasta.