1. Combine 1 cup boiling water and porcini mushrooms in a bowl; let stand 20 minutes. Drain in a colander over a bowl, reserving 1/2 cup soaking liquid. Chop porcini.
2. Cook pasta in boiling water with 2 teaspoons salt until almost al dente; drain. Rinse pasta with cold water, and drain.
3. Heat a large saucepan over medium-high heat. Add 1 tablespoon oil, chicken, 1/4 teaspoon salt, and pepper; sauté 5 minutes, turning to brown. Remove chicken. Melt butter in pan. Add cremini mushrooms and garlic; sauté 3 minutes, stirring frequently. Add flour; sauté 1 minute, stirring constantly.
4. Arrange rack in middle of oven. Preheat broiler to HIGH.
5. Combine reserved porcini liquid, broth, and milk. Gradually add broth mixture to pan, stirring constantly; bring to a boil. Reduce heat, and cook 1 minute or until slightly thick, stirring frequently. Stir in processed cheese and cream cheese. Remove from heat; stir until smooth. Stir in Gruyère. Add porcini, pasta, chicken, peas, and chives; toss. Scrape pasta mixture into a broiler-safe 13 x 9-inch ceramic baking dish.
6. Place bread in a food processor; drizzle with remaining 1 tablespoon oil. Process until coarse crumbs form. Sprinkle crumbs over pasta mixture. Place dish on middle rack in oven; broil 6 minutes or until golden.