Heat the butter and oil in a pan over a medium heat. Gently fry the onion and garlic for 3-4 minutes, or until softened.
Pour the stock into the pan and bring to a simmer, then stir in the cooked rice. Simmer for a further five minutes, then stir in the spinach until wilted. Stir in the cream.
Blend the mixture using a hand-held blender until smooth.
Meanwhile, for the croûtons, place the bread onto a baking sheet, drizzle over the oil and season, to taste, with salt and freshly ground black pepper. Bake for 5-10 minutes, or until golden-brown and crisp.