Preheat oven to 400°. Peel sweet potatoes and cut into 1-inch cubes. Seed red pepper and dice into 1-inch pieces. Halve tomato, remove stem and chop into quarters.
 
Spread sweet potatoes, peppers and tomatoes evenly on a parchment paper-lined baking sheet. Sprinkle with sea salt and chili powder. Roast for 20 minutes or until sweet potatoes are fork tender. Allow vegetables to cool. In a food processor combine roasted vegetables with nutritional yeast and lemon juice. Process until smooth and velvety. If the queso is a little thick, add water a spoonful at a time to reach the desired consistency.
 
In a small saucepan over low heat, warm queso, then transfer to a serving bowl. Garnish with scallions and cayenne. Serve queso dip with blanched broccoli florets, fresh celery sticks and tortilla chips.