Place rack in centre of oven. Preheat oven to 200°C (400°F). Line a baking sheet with parchment paper.
For the squash: Cut in two lengthwise. Remove seeds with a spoon. Oil, salt and pepper the squash, then place cut side down on the baking sheet.
For the stuffing: Cook the quinoa for 12 to 15 minutes in a saucepan of boiling salted water. Rinse under cold water and drain well. Set aside. In a frying pan, brown the onion and garlic in oil. Add the quinoa, pepper and raisins. Reheat over medium heat, stirring frequently. Add 125 ml (1/2 cup) of Cheddar and coriander, if desired. Season to taste.