Cook beef in large non-stick skillet over medium-high heat for 8 to 10 minutes, breaking up and stirring with wooden spoon, until browned throughout and fully cooked. Drain if necessary. Add onion, garlic, chili powder, oregano, cumin (if using) and red pepper flakes; cook until onion is tender, about 5 minutes.
Stir in milk and tomato paste, mixing well; simmer for 5 to 10 minutes or until most of the liquid is absorbed.Season with salt and pepper to taste.
Meanwhile, prep Toppings and heat taco shells upside down on baking sheet in oven for 3 to 5 minutes. To serve, spoon beef mixture into each shell and add Toppings and Guacamole.
Guacamole Dip: In medium bowl, mash the flesh from 2 ripe avocados using a potato masher until fairly smooth. Add 1/3 cup (75 mL) 10% half and half cream, 1/4 cup (50 mL) light mayonnaise, 1/2 tsp (2 mL) grated lime rind and 1 tbsp (15 mL) fresh lime juice; stir until smooth and well combined. Stir in 1 small Roma (plum)tomato (seeded and diced), 1 clove garlic (minced)and a dash of hot pepper sauce. Season to taste with salt. Garnish with chopped fresh coriander.