
Make the croutons: Place the bread on a baking sheet. Bake at 350F until golden, about 8 to 10 minutes. Toss with rosemary, 2 tablespoons oil and garlic.
Heat 2 tablespoons oil in a large skillet over medium-high heat until almost smoking. Add the mushrooms and remaining onions. Cook, stirring occasionally, until the mushrooms are golden, 5 minutes. Add the vinaigrette, stir well and remove from heat. Immediately add to the greens and toss once. Add the nuts, prosciutto and croutons; toss vigorously. Divide among plates. Serve this seared mushroom salad immediately.
Adapted slightly from Jonathan Waxman’s A Great American Cook.
I also added cherry tomatoes but not necessary, especially if it's winter and there are no good cherry tomatoes to be had.