1. Combine mayonnaise, oil, lemon juice, parsley, scallion, tarragon, garlic, salt, sugar substitute and pepper in a gallon-size resealable plastic bag. Add chicken, seal bag and shake to coat. Refrigerate 1 hour.
2. Heat a charcoal or gas grill to medium or heat a grill pan. Grill chicken, turning occasionally, until just cooked through, about 7 minutes.
1. Over an open flame on the stove, or under a broiler, roast peppers until skins are charred on all sides. Place in a paper or plastic bag and seal bag. Let peppers steam 10 minutes. When peppers are cool enough to handle, peel, seed and cut into 1/3-inch-wide strips.
2. Transfer to a glass dish, drizzle with olive oil. Peppers will keep refrigerated for up to a week.