In a blender, combine garlic, rosemary, oregano, mustard and vinegar and blend until garlic is mixed well.
While machine is on (keep cover on, removing the smaller opening at top), gradually add olive oil and blend until creamy.
Season lightly with salt and pepper, between ½ - 1 teaspoon of kosher salt and a little pepper.
If making immediately add the meat to a glass dish, scoring slightly on both sides, pour all but about ½-¼ cup vinaigrette over meat. Lift to coat all sides of the flank steak.
Allow to marinate in the fridge for 4 - 24 hours.
If freezing, place meat in freezer baggie (don't forget to write the instructions and what it is on the baggie).
Pour all but ½ - ¼ cup vinaigrette over meat, squish around coating all sides and remove air before sealing tightly.
Pour leftover vinaigrette into small jar and place in fridge, removing from fridge about 30 minutes before serving.
Lay flat in freezer.
When ready to cook, remove from freezer and place in fridge overnight or on counter for 2 hours.
Heat grill to medium and lightly oil grill.
Remove steak from marinade, allowing excess to drip off.
Place steak on grill and grill for 6-8 minutes per side (depending on thickness and desired doneness).
If desired, baste or pour vinaigrette left in baggie or dish onto meat while cooking.
Remove from grill and allow to rest on plate for 5 minutes, loosely covered with foil.
Slice against the grain and drizzle with a little remaining vinaigrette (the one from the fridge, not what was on the meat).
Serve immediatly.
Pair with a crispy salad drizzled with vinaigrette, roasted veggies (also drizzled with vinaigrette), juicy watermelon and a flaky biscuit.