Wash the vegetables and dry them well.
Preheat the oven to 450 degrees and line a large baking sheet with aluminum foil.
Use a sharp chef’s knife to cut and seed the bell pepper, slicing it into large 2 inch chunks. Next, slice the zucchini into 1 inch thick round pieces. Cut the mushrooms into large, bite-sized pieces. Peel the sweet onion and slice the onion into 2 inch chunks.
Leave the tomatoes whole and transfer all the vegetable pieces into a large mixing bowl. Use a large wooden spoon to toss the veggies, mixing them well.
Drizzle the oil over the vegetables, tossing them to coat them in oil.
Using a mortar and pestle or a spice grinder, crush together the salt, coriander seed, and mustard seed together. Blend and crush the spices until the peppercorns have all been broken. Add the rest of the seasonings and blend them together until they are well combined. Transfer the seasoning mix to a small bowl with a spoon.
Use the spoon to sprinkle the seasoning mix over the vegetables. Use your wooden mixing spoon to toss the veggies and continue adding the seasoning until they are lightly coated in the seasoning mixture.
Spread the vegetables on the aluminum foil covered pan, allowing plenty of space between the pieces of veggies and spreading them in an even layer.
Put the baking pan into the preheated oven and bake the veggies for about 10 minutes. Stir the vegetables and spread them in an even layer on the pan. Return the pan to the oven and bake them again for 10 to 15 more minutes. The veggies should start browning and getting a bit crispy. Stir them again and return the veggies to the oven until they are golden brown and slightly crisp or about 5 minutes.
Take the pan from the oven and transfer the vegetables to a serving bowl. Serve the veggies hot from the oven.