Melt butter in a large non-stick saucepan over medium heat. Add celery, onion, carrot, capsicum and thyme. Sauté until vegetables are tender, 10 minutes.
Stir in the flour until thoroughly combined. Stir in stock. Increase heat to medium-high. Add broccoli, cauliflower and onion. Bring to the boil. Lower heat and simmer, uncovered, 15 minutes. Remove pan from heat. (If making filling ahead, leave it to cool to room temperature. Cover and refrigerate.)
Preheat oven to 200 degrees C. Spread the vegetable mixture in a 23cm glass or ceramic pie plate. Top with cheddar. Cover with pastry (see Hint). Trim edges. (If the crust is not a lattice, cut 6 long slits in it to let the steam escape.) Place the plate on a baking tray.
Bake until pastry is golden and filling is bubbling, 25 to 30 minutes. Let stand 10 minutes before serving.
To make a lattice pie crust, roll out dough 10 or 15cm wide and longer than the pie plate by 5cm. Cut into thin strips (10 or 12 will be enough). Starting at the edge of the plate, weave the strips over the pie, lifting up those made in one direction where necessary, to enable you to weave strips in the other direction. Neatly trim the ends.
I made it into 4 smaller, single-serve pies, for ease. It meant I could make up the mixture, use half straight away for 2 pies, then freeze the rest of the mixture for the following week to make the other 2 pies. Great for when there's only 2 of us to cook for. Yes the mixture freezes/defrosts well. - 13 Jul 2010
This pie is fantastic! I was surprised at how creamy the sauce the vegetables were in became without the addition of cream, only flour (which is so much healthier). I'm not much of a fan of cauliflower, but in this dish it tasted great. - 18 May 2009