Complex sentence: Shortly thereafter, Surmain became partners with Soltner and they ran the restaurant together until Surmain returned to Europe, first retiring to Majorca, later running "Le Relais à Mougins" in Mougins, Southern France.
Split into simpler sentences: Shortly thereafter, Surmain and Soltner became partners, and they ran the restaurant together until Surmain returned to Europe. He first retired to Majorca, then later ran "Le Relais à Mougins" in Mougins, Southern France.