Combine lemon juice, mustard, oil, garlic and thyme in large sealable bag. Pierce steak all over with fork; add to bag, seal and refrigerate for 1 to 2 hours for Grilling Steak or 8 to 12 hours for Marinating Steak.
Discard marinade. Pat steak dry with paper towel; season lightly with salt and pepper.
Grill, broil or pan-fry using medium-high heat, turning twice or more with tongs. Cook until digital instant-read thermometer inserted sideways into steak reads 145ºF (63ºC) for medium-rare, or 160ºF (71ºC) for medium. Let rest for 5 minutes. Serve with Sage Onion Compote.