Boil potatoes just lightly until about cooked.
Place chopped veggies and garlic in a casserole dish or saucepan with 1 tablespoon oil and sweat veggies for 5 minutes. (This means to put them (preferably) in a covered frying pan and let cook on low for about 5 minutes -- just long enough to take a bit of the crunch off, but not enough to cook them entirely.)
Do not overcook.
Add the herbs and seasonings to the vegetables.
Mix mustard powder, flour and tomato paste with water and stock until smooth.
Pour mixture onto the vegetables and bring to a boil, stirring constantly.
Season with salt and pepper.
Transfer to an ovenproof dish if necessary.
Place the potato in a bowl with the remaining 2 tablespoons of oil and toss until coated.
Season with salt and pepper.
Pile the potatoes on top of the veggies and bake in a preheated oven at 400 F for 45 minutes.
The potatoes should be brown, but if they're not you can grill them for another 5 minutes.
Serve with a salad or on top of rice.
