In a bowl, toss together the bread, cheese and spinach. Whisk the remaining ingredients together in a jug and pour over the bread mixture.
Cover with cling film and chill in the fridge for at least 2 hours or overnight.
Preheat the oven to 180°C, fan 170°C, gas 4. Line a 12-hole muffin tray with 10 paper muffin cases and divide the muffin mixture evenly between them. Bake in the oven for 20-25 minutes or until set.
Let the muffins cool slightly in the tin before removing. Serve warm.