Combine 3/4 cup water and sugar in a medium saucepan. Bring to a boil, stirring until sugar dissolves. Stir in rhubarb and thyme; return to a boil. Cover, reduce heat, and simmer 8 minutes or until rhubarb is tender. Place pan in a large ice-filled bowl for 10 minutes or until cool, stirring occasionally. Discard thyme sprigs.
Place rhubarb mixture, strawberries, and lemon juice in a blender; process until smooth. Pour mixture into an 8-inch square glass or ceramic baking dish. Cover and freeze until partially frozen (about 1 hour). Scrape with a fork, crushing any lumps. Freeze, scraping with a fork every hour, 4 hours or until completely frozen.