A quick spin in the processor transforms long-simmered onions into a silken puree.
Tie thyme sprigs with kitchen string. Melt butter in a large skillet over medium-high heat; add olive oil; onions, and thyme sprigs, stirring to coat. Sprinkle with salt, and reduce heat to medium-low. Cover and cook, stirring occasionally, 1 hour or until onions are tender. Discard thyme sprigs. Drain onions, using a fine wire-mesh strainer and reserving liquid for another use. Process onions in a food processor 30 to 60 seconds or until smooth. Serve immediately.