Step 1Wash the green beans and dry well. Place in a large bowl and add 1 tablespoon of the olive oil and half of the salt. Season with pepper to taste. Toss to coat well. Spread on a rimmed baking sheet.
Step 2Cut the red pepper and onion into slices about the same thickness as the green beans. Place in a bowl and add the remaining oil and salt and season with pepper. Toss to coat well. Spread on a second rimmed baking sheet.
Step 3Heat the oven to 475 degrees. As it heats, toast the walnuts in a small baking dish, 5 to 10 minutes; watch carefully to be sure they don't burn. Cool, then coarsely chop.
Step 4Roast the vegetables until they're soft and starting to caramelize, 15 to 30 minutes, stirring and turning every 5 minutes to cook evenly without burning.
Step 5Transfer to a shallow serving dish. Sprinkle with the walnuts. Crumble the blue cheese over and sprinkle with the chives. Serve warm or at room temperature.