In a colander, toss the cucumbers and bell pepper with 1/2 teaspoon salt. Let stand at room temperature for 30 minutes to drain. In a glass or stinless steel bowl, whisk the lemon juice with the mustard. Whisk in the oil and chives; season with salt and black pepper. Transfer the cucumbers and bell peppers to a medium glass or stainless steel bowl. Add the lemon dressing and toss to combine.