Hummus Layer. Use a 9-inch by 7-inch by 2 ½-inch (11 cup) rectangular baking dish. Spread the hummus evenly over the bottom of the dish.
Red onion layer. Put the onions into a colander or strainer and rinse well under cool water. Shake dry and put into a small bowl. Stir in 1 Tbsp of the lime juice, cilantro and ¼ tsp of the salt. Arrange in a single layer over the hummus in the dish.
Sour cream layer. Mix the sour cream with ¼ tsp salt. Spread in a layer over the onions.
Tomato layer. Pat the tomatoes dry and mix with 1 tsp salt. Arrange tomatoes over sour cream.
Guacamole layer. Scoop the avocado pulp out into a medium-size bowl. Coarsely mash with a fork. Stir in salsa, arugula, remaining 1 Tbsp lime juice and about 1 tsp salt. Gently spread the arugula guacamole over the tomatoes. Sprinkle with bacon and cheese. Refrigerate up to 2 hours before serving. Serve with thick tortilla chips, sliced jícama or sliced cucumbers.