Heat oil in a large non-stick saucepan over medium-high heat. Cook onion, stirring occasionally, until soft and starting to turn golden, 5 to 8 min.
Add spinach and cook until wilted, about 2 min. Set aside in fridge to cool completely, about 15 min.
Place spinach mixture in bowl of food processor. Add mayonnaise, sour cream, lemon juice, salt and pepper. Blend until smooth, about 30 sec.
To serve spinach dip the classic way, hollow out a large round pumpernickel or sourdough loaf, leaving a 1-in. (2.5-cm) thick "bowl". Fill bread bowl with dip. Dice remaining bread and use for dipping. Alternatively, serve with cut-up fresh vegetables: carrots, cucumber, celery, radishes.