In a medium saucepan, bring 2 quarts of salted water to a boil.
Set up a bowl with ice water and place near the sink.
Place asparagus in boiling water and leave in for 2 to 3 minutes or until barely limp. Immediately transfer asparagus to ice bath for 5 minutes to stop the cooking process.
Meanwhile, in a medium saute pan over medium-high heat, add olive oil, mushrooms and garlic. Cook about 2 minutes or until mushrooms are beginning to brown.
Add tarragon and salt, pepper and a splash of vinegar. Set aside.
On 6 salad plates, plate 4 or 5 spears of asparagus. Top with a mound of arugula and arrange 2 or 3 slices of tomato on each mound.
Drizzle porcini dressing over each salad and serve.