Sauté onion and garlic in butter and olive oil until softened.
Add rice, stir to coat, then add chicken stock and bring to a boil.
Simmer covered for 15-18 minutes until stock is absorbed and rice is tender.
Trim fat and sinews from kidneys and cut into bite-sized lobes.
Sear kidneys in smoking hot olive oil for 3-4 minutes then drain in a colander.
Fry shallots in butter for 5 minutes, then deglaze with brandy and wine, reducing both by half.
Stir in cream and reduce to a light sauce before whisking in the mustards.
Add kidneys to the sauce and reheat gently without boiling.
Sauté garlic in butter, add spinach, and cook until wilted with a splash of water.
Serve kidneys and sauce over rice with a side of wilted spinach.