Add oil to and heat a 12-inch skillet over medium high heat until oil is hot. Add asparagus, leeks and salt and saute turning frequently until just getting tender, about 5 minutes. Remove asparagus and leeks to a plate.
Return skillet to heat, and melt butter. Add spinach and peas, season with remaining ½ teaspoon salt and the pepper; cover, and cook until spinach is wilted and peas are tender, about 3 minutes.
Return the asparagus and leeks to the pan. Nestle a seared steak in the center and serve immediately.