Preheat oven to 400°F. Heat oil in a large ovenproof skillet over medium-high heat.
Chop the onion and garlic and saute in a frying pan in a glug of olive oil until soft and fragrant.
Dice the chorizo into chickpea-sized chunks and add to the pan.
Saute everything together until the orangy paprika oils run from the chorizo.
Add the spinach and keep stirring until it wilts and everything starts to meld together in the pan.
Whisk together eggs and add salt. Pour over vegetable mixture in skillet and stir gently to incorporate the eggs into the whole mixture, then allow to set over a medium heat. Cook 2 minutes or until edges begin to set.
Preheat the grill then slide the whole pan under the grill to set the top of the fritatta, it will only take a minute or two to become light gold and puffy.
Run a heatproof spatula around edges of frittata to loosen, then slide onto a plate. Allow to cool slightly before slicing into canape sized cubes or wedges. Serve hot or room temperature as a tapas or with a salad.