Preheat a large pan coated with olive over high heat and sear the pieces of beef to form a nice golden crust on all sides.
Add onions, garlic, salt, pepper, lower heat to medium and continue cooking until the onions are softened.
Add 2 cups of water, Dijon mustard and fresh rosemary and stir until the mustard is well incorporated.
Cover loosely, lower heat and simmer until the liquid is almost completely evaporated then add another 2 cups of water and reapeat the process.
At this point, you want to add your mushrooms, pearl onions and another 2 cups of water. Bring back to a simmer and continue cooking until you're left with a nice, thick sauce; Turn off the heat.
In a mixing bowl, add yogurt, arrowroot and about a cup of your cooking liquid. Mix with a wisk until fully combined and add that to the pan.
Add fresh parsley and stir delicately until all is well combined.
Serve over zucchini ribbons
Notes
*The use of a mandolin is strongly recommended to get the zucchini slices really paper thin. Alternatively, you could also cut the zucchini into long thin strips to mimic spaghetti.