A musical polymath, David Owen Norris was already well-known in his native England for his piano broadcasts and TV commentaries when he was named last year as the first Gilmore Artist. Valued at $250,000, the award--given by Kalamazoo's Gilmore Festival--is intended to promote a touring career by subsidizing much of the fees and expenses. Norris is a bit old at 38 to be in the company of precocious winners of international competitions, but his introspective musicianship projects intelligence at the keyboard, and his tastes reveal a fondness for the unconventional and for the much-maligned modern British repertoire. At Ravinia he'll demonstrate his skills both as a soloist and as an accompanist. Tonight he makes his debut with the Chicago Symphony Orchestra under the direction of Gennady Rozhdestvensky in Ravel's dapper Piano Concerto in G. (Mozart's Serenade no. 4 and Mendelssohn's Italian Symphony make up the remainder of the program.) On Monday he will accompany noted Swiss soprano Edith Mathis in her first Ravinia appearance in a bountiful program of songs by Mozart, Brahms, Strauss, and Duparc. Tonight (pavilion) and Monday (Murray Theatre), 8 PM, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 728-4642.