additional lemon juice, salt, and freshly ground black pepper to taste
Scrub potatoes well and cut into bite-size pieces. (Halve very small potatoes and quarter larger ones.) Bring a pot of water to a boil, add potatoes, and cook until just tender but not falling apart (10-20 minutes, depending on size of potatoes). When done, drain and rinse briefly with cold water.
Meanwhile, prepare the green beans. Trim ends and cut beans into bite-sized pieces, 1 to 1 1/2-inches long. Bring a pot of water to a boil, add the green beans, and cook until tender but still bright green and crisp, 5 to 10 minutes. (Or steam in pressure cooker: place in steamer basket over boiling water, seal cooker, and cook for 3 minutes; use quick release to bring pressure down.) Drain water from beans and immediately plunge them into a bath of ice and water to stop cooking.
To make the dressing, place 2 ounces of the cooked potatoes (60 g) into a small food processor or mini blender. Add basil, lemon juice, garlic, miso, and 1 tablespoon water. Process until creamy, scraping down sides and adding more water if necessary. Drain the green beans and add them to the potatoes in a large serving bowl. Gently fold in the dressing. Cover and refrigerate for at least 2 hours, to allow flavors to blend.
Before serving, add the sliced red bell pepper. If you like, you can roast the strips of pepper briefly under a broiler to soften and sweeten their flavor. Taste for seasoning and add additional fresh lemon juice, salt, and freshly ground pepper as needed.