In an extra-large skillet cook beans in boiling lightly salted water about 3 minutes or until crisp-tender; drain. Transfer to a bowl of ice water to stop cooking. Drain again; set aside. Meanwhile, in a small bowl pour the 1 cup boiling water over the dried mushrooms. Cover and let stand for 15 minutes.
In the same extra-large skillet cook bacon until crisp. Using a slotted spoon, transfer bacon to a small bowl lined with a paper towel; crumble bacon. Reserve about 1 tablespoon of the bacon drippings in skillet; discard the remaining drippings. Cook cremini mushrooms in the reserved drippings over medium-high heat until lightly browned, stirring occasionally. Stir in garlic. Cook and stir for 1 minute more. Remove from heat. Stir in green beans.
Meanwhile, use a fork to remove the dried mushrooms from the water (do not discard liquid). Chop the mushrooms; add to green bean mixture.
For sauce, in a medium saucepan melt butter over medium heat. Stir in flour. Cook and stir for 1 minute. Stir in all but about 2 tablespoons of the mushroom liquid (discard the liquid at the bottom of the bowl, which may be gritty). Stir in half-and-half. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Remove from heat. Stir in rosemary, salt, and pepper. (To serve today, omit Step 5 and continue as directed in Step 6.).
Transfer crumbled bacon, green bean mixture, and sauce to separate airtight containers. Cover; chill for up to 24 hours.
Preheat oven to 375 degrees F. Stir the sauce into the bean mixture. Spoon into a 2-quart square baking dish. Bake for 25 to 30 minutes or until bubbly. Top with bacon. Bake for 5 minutes more.