Evan Kleiman, a young American chef, has a gift for maintaining the natural, unpretentious, sensual essence of Italian cooking. And that is what sets her apart as one of the sharpest interpreters of Italian cooking in Los Angeles. A case in point: Kleiman's interpretation of Arista , a Tuscan pork roast seasoned with fennel, shown here. Traditionally, this roast is served with the bone in, causing the darker meat of the loin to infuse the leaner tenderloin with its juiciness. Kleiman's version uses a boneless pork rib roast, which allows, she says, "for more thorough seasoning and easier slicing when serving." The chef is an advocate of foods eaten at room temperature, not piping hot, an idea foreign to most Americans. The taste sensation of foods allowed to mellow at room temperature could be a revelation; it's worth a try, if only to relieve pressure from the cook harried by timing. Kleiman recommends serving Arista with sauteed rapini . (That's what it's called in Italian. In English, it's called rape, or broccoli rape because of the tendency of its elongated leaves to develop blossoms resembling those on broccoli flowers.) Rapini , an annual Old World plant of the mustard family of greens, was grown in backyard and tenement-window gardens for home use by early Italian immigrants who were homesick for the vegetable. Later, the plant made rare appearances at European ethnic markets and could be found at Oriental grocery stores as well. Now that rapini has become fashionable, a shopper can find the green at almost any gourmet food store that carries produce. Rapini can be served as a vegetable accompaniment, warm or at room temperature, and is delicious as a sandwich filling.