Heat oil in a large nonstick or cast-iron skillet over medium-high, add the gnocchi in an even layer, and cook covered until golden and crisp underneath, about 2–4 minutes.
In a small bowl, mix butter, miso, vinegar, and black pepper until smooth.
Add asparagus and a pinch of salt to the skillet, cook 2–3 minutes until bright and tender, then remove from heat.
Stir in the miso butter until melted and the gnocchi are coated, season with salt and pepper, and toss in the arugula until just wilted.
Serve immediately.
