Toss strawberries and green onion with sugar and salt and let sit for 30 minutes. Strain off any liquid that comes out into a small pot. Add balsamic and simmer until a glaze consistency. Add glaze to strawberries and stir in tarragon and chili flakes. Taste for seasoning and chill until ready to serve.
In a food processor, pulse onion, garlic vinegar, mustard and pepper until smooth. Drizzle in oil while processor is running, and pour marinade into a shallow dish. Place sirloin into dish and turn to coat with marinade. Cover and chill for at least 4 hours, turning occasionally.
Preheat grill to medium heat. Remove all but a thin layer of marinade over beef and season with salt. Place on grill and cover. Grill on one side, rotating once, about 12 minutes, then turn and grill on the other side about the same time for medium-rare (use a temperature probe and grill to an internal temperature of 135°F). Let sirloin rest for 5 minutes before slicing.
Any reserve marinade can be simmered for 5 minutes, then served on the side as a “relish” beside the spring salsa (or toss into boiled potatoes).