Preheat the oven to 425 degrees. In a large bowl, toss the cauliflower and onion with 3 tablespoons of the olive oil and season with salt and pepper; spread the vegetables on a large baking sheet. In the same bowl, toss the beans with the remaining 1 tablespoon of olive oil and season with salt and pepper. Spread the beans on a small baking sheet. Roast all of the vegetables until crisp-tender and lightly browned, stirring once or twice, about 14 minutes for the beans and 16 for the cauliflower and onions. Return all of the vegetables to the bowl and let cool.

Meanwhile, in a medium skillet, heat the vegetable oil. Add the bread and cook over moderate heat, stirring, until golden and crisp, about 5 minutes. Transfer to a plate to cool and season with salt.

In the same skillet, toast the curry powder over moderate heat, stirring constantly, until fragrant, about 2 minutes. Scrape the curry over the vegetables, season with salt and pepper and toss to coat.

In a small bowl, combine the yogurt with the lemon juice, cilantro, and garlic and season with salt and pepper. Sprinkle the vegetables with the croutons and serve with the yogurt sauce.

The roasted curried vegetables and lemon-yogurt sauce can be refrigerated separately overnight. The croutons can be kept in an airtight container at room temperature overnight.