Trim off about an inch off the bottom of the asparagus and then cut them into 2 or 3 pieces, depending on their length. You want pieces of approximately 3” long.
Cut the sweet bell peppers in half and remove the stems and seeds, then slice the onion fairly thinly.
In a glass jar or small mixing bowl, add olive oil, apple cider vinegar, Dijon mustard, salt, pepper, lemon zest, capers and fresh herbs then mix well with a flat whisk until slightly emulsified.
Pour about 3/4 of the vinaigrette over the vegetables and stir well.
Roast the vegetables in the oven for about 15 minutes, stirring once or twice, until they are softened and start to brown, then set the oven to broil and let the vegetables caramelize for 5-6 minutes.
Remove from the oven, pour the rest of the vinaigrette over the vegetables, add the chopped walnuts and stir well.
Serve immediately or allow to cool to room temperature.