In medium skillet over medium heat, combine onions, olive oil and sugar. Cook, stirring often, and lowering heat as liquid cooks away, until onion is tender and deeply golden brown (caramelized); set aside.
In heavy saucepan over medium heat, melt butter. Add shallots and garlic and cook, stirring, until softened, 2 to 3 minutes.
Add mushrooms and cook, stirring, for 3 to 4 minutes longer. Add rice and stir for a minute or two until “milky” in appearance. Add wine and stir until it has mostly cooked away.
Add one cup of hot stock and cook, stirring often, until it is nearly all absorbed. Continue adding stock one cup at a time, letting it be absorbed each time and adjusting heat to maintain gentle simmer.
When rice has absorbed enough stock to be creamy and tender but still slightly firm in center (this should take about 20 minutes), stir in cheese and herbs. Stir in chicken and reserved onions.
To serve, place spinach around edge of shallow bowls and spoon risotto in center.