Prepare the Vegetables: Preheat your oven to 375°F (190°C).
In a small bowl, stir together the cumin, coriander, salt, and pepper.
Season and Roast: In a large bowl, toss the zucchini, bell peppers, and onions with 2 Tbs. olive oil.
Sprinkle the prepared spice mixture over the top and toss again until the vegetables are evenly coated.
Bake to Perfection: Divide the vegetables between two baking sheets, spreading them in a single layer.
Roast for 30 to 45 minutes, turning the vegetables once or twice and rotating the baking sheets halfway through, until tender and golden brown.
Set aside to cool.
Make the Bean Spread: While the vegetables roast, heat 1 tsp. (5 ml) olive oil in a small skillet over medium heat.
Add the minced garlic and cook for 30 to 60 seconds, until fragrant.
Add the cannellini beans and coarsely mash them in the pan with a fork or potato masher.
Stir in the water and cook for about 10 minutes, stirring occasionally, until the mixture thickens to the consistency of refried beans.
Remove from the heat, stir in the lemon juice, and let it cool.
Assemble the Sandwiches: Spread about 2 Tbs. of the bean spread onto each slice of bread. On four of the slices, layer 1 cup (240 ml) of the roasted vegetables, 2 or 3 tomato slices, and ½ cup of arugula.
Place the remaining four bread slices on top, cut in half, and serve immediately.
Don't overcrowd the pans when roasting the vegetables.
Spreading them in a single layer ensures they roast and caramelize rather than steam, giving you the best flavor and texture.