On a large rimmed baking sheet, toss the bread with 1/4 cup of the olive oil and season generously with salt and pepper. Spread in an even layer and bake for about 12 minutes, until golden and almost crisp. Transfer to a bowl and let cool. Keep the oven on.
Meanwhile, in a large bowl, toss the carrots, 2 tablespoons of the olive oil, and the coriander and cumin seeds. Squeeze the orange and lemon over the carrots, then add the squeezed fruit to the bowl, too. Season generously with salt and pepper and toss to mix. Spread on two large rimmed baking sheets and roast for about 25 minutes, until lightly charred and tender. Let cool; discard the orange and lemon.
In a small bowl, whisk together the vinegar, shallot, mustard, and the remaining 6 tablespoons olive oil. Season the vinaigrette with salt and pepper.
On a large serving platter or in a serving bowl, gently toss the bread, carrots, and avocados with the vinaigrette. Add the watercress, season with salt and pepper, and toss to mix. Serve at room temperature.
Can't find watercress? You can substitute baby arugula, which has a similar peppery bite that contrasts with the sweetness of the roasted carrots in this salad.