DINING: Chef Ron Siegel prepped on Wednesday morning for that evening's dinner at his new upscale, Japanese-infused venture in the Western Room at Rancho Nicasio. Mr. Siegel—who recently left his post as executive chef at San Francisco’s Michael Mina, where a prix fixe dinner costs $125—said he approached Rancho owner Bob Brown about a potential partnership at a show many moons ago.   David Briggs