MmmmMMMmmmm! This was so good! I have to say, I was skeptical - a VEGETARIAN Reuben? Pshaw, sure, whatever lady. *smile* My husband and I, both, love Reubens, but I halved this recipe, and just made it for myself. My husband is not too keen on mushrooms. Plus, I learned at the grocery store yesterday that the idea of eating one AS a sandwich was totally crazy/borderline punishment to him. Haha. Anyway, I reduced the heat on the onions to medium-low when I reduced it. Any lower than that, and I think it might take forever to reduce. For the mushroom marinade, I put a ziploc bag into a clean glass, and draped the zippy part over the rim (I included a picture - this made it easy to put liquids into a baggie). I am not sure how long I marinated my mushroom - haha that sounds funny - but I put it in the fridge and just took it out when the onions were almost done, so I could grill it. I used my stovetop grill pan and grilled it for about 3 1/2 minutes on each side. I only put mayo on one slice of bread (I'm not really a fan of it), and I didn't really think it needed it, personally. I would leave it off next time. I had a bunch of onion marmalade leftover. It would be very good on teriyaki chicken sandwiches - if you're not totally into the vegetarian thing. The flavors of this are definitely tangy and meaty, very much like a Reuben! I am keeping this recipe, and I would make it for my mom when she comes over for lunch. I'm glad I picked this one - sorry for the honkin' long review! Made for Spring PAC 2011.