Break up crackers into a food processor. Process until very finely crushed. Add the butter and pulse to combine. Press the mixture over the base and side of a 3cm deep, 23cm (base measurement) round fluted tart tin with a removable base. Place in the fridge for 15 minutes to chill.
Meanwhile, heat the oil in a frying pan over medium-high heat. Add the garlic and cook, stirring, for 1 minute or until aromatic. Add the spinach. Cook, stirring often, for 5 minutes or until the spinach has defrosted and any excess liquid has evaporated. Set aside until required.
Preheat oven to 180C/160C fan forced. Bake the crust for 10 minutes or until golden. Set aside to cool slightly.
While the crust is cooling, use electric beaters to beat the cream cheese until smooth. Beat in the sour cream and milk until combined. Add the eggs, one at a time, beating well after each addition. Stir in the spinach mixture, shallot and mint. Season.
Pour the cream cheese mixture into the baked crust. Sprinkle with feta and cheddar. Bake for 20 minutes or until filling is set. Set aside to cool for 1 hour or until quiche is room temperature.
Sprinkle with green shallot to serve.
