In a large, non-stick skillet, melt butter over medium heat. Add in diced yellow onion and sauté until lightly browned and tender, 6-7 minutes.

While onions are cooking, scoop avocados out into a large bowl and mash together with lime juice, garlic, salt and pepper. 

Stir in browned onions, diced tomatoes and cilantro. Serve with tortilla chips.

To store in the fridge, scoop guacamole into an airtight container, press plastic wrap directly on top of it and cover with container lid.