In a medium bowl, whisk together 6 eggs, half & half, water, sea salt, and freshly ground black pepper until blended. Set aside.
Add olive oil to a 10” non-stick skillet and heat over medium heat. Add Portobello mushroom slices and chopped stems and cook approximately 5-6 minutes, stirring occasionally. When the mushrooms are browned, remove 3 uniformly sized mushroom slices from the pan and set aside.
Add arugula to pan and stir constantly until wilted, approximately 1-2 minutes.
With a spoon, spread ingredients into a uniform layer across the bottom of the pan, ensuring an even distribution of mushrooms and arugula throughout. Pour egg mixture on top and let cook for 1-2 minutes, or until the egg begins to set on bottom.
Reduce heat to just above the “low” setting and cover. Cook 8-9 minutes, then remove cover and arrange the 3 Portobello strips, cherry tomato halves, and Feta chunks in a decorative pattern, if desired. If not, simply scatter those ingredients across the top of the frittata before covering again.
Cook for another 5-6 minutes, or until the egg is cooked through and the Feta has just begun to melt. Remove from heat and carefully transfer to a serving platter and cut into wedges.
Serve warm or enjoy at room temperature.