In a large oven-proof skillet or cast-iron skillet, heat oil for over medium-high heat. Add scallions and sauté until fragrant, about 1 minute.
Add asparagus and broccolini and stir well to cook until tender, about 2-3 minutes.
Stir in peas and spinach with seasoning. Stir well until evenly combined, seasoning is toasted and aromatic, and spinach is wilted. Remove from heat.
In a large mixing bowl, whisk together eggs and cream until smooth. Pour the egg mixture into the skillet and gently shake to distribute. Sprinkle feta cheese evenly on top.
Bake uncovered for 20 minutes or until completely set on top. Let the frittata rest for 5-10 minutes. It will puff in the oven and deflate a bit while cooling. Garnish with chopped herbs and extra feta on top, if desired. Slice and serve.