Make the Tarragon Emulsion: In a pot combine the shallots, olive oil and salt and cook on medium heat, covered until completely soft with no color. Cool in a bowl over ice, then place in blender with the tarragon and blitz until completely smooth. Add water to thin the emulsion as necessary.
Cook and glaze the Beef Flank: Heat a medium pan with a splash of oil on a moderate heat. Add the Beef Flank and sear on all sides, until a crust forms. Add enough glaze to coat the Beef Flank, reducing the temperature and continually tossing the Beef Flank until cooked through and coated in the sticky glaze. Remove from the pan to rest for one minute.
Assembly: Place the Beef Flank cubes onto your serving dish, placing a skewer into each cube. Top with tarragon emulsion and garnish with crispy onions.