To Make the Pesto:
1. Heat ½ tablespoon olive oil in a small skillet over medium-high heat. Add chopped ramp bulbs with a pinch of salt and cook until they start to soften. Add ramp greens and cook until slightly wilted, about 2 minutes.
2. Transfer ramps to food processor. Add remaining ¼ cup olive oil, walnuts, garlic, lemon juice, and Parmesan. Pulse until smooth. Adjust lemon juice or Parmesan to taste.
To Make Potatoes and Fish:
1. Preheat oven to 400F.
2. Toss potatoes in olive oil and Kosher salt. Roast for 20 minutes. Remove from oven and place cod pieces on top. Spread pesto on top of cod and return to oven for 20 minutes.
3. Spritz with lemon juice before serving.