Mix the taco seasoning ingredients together and set aside.
Add 1 tbsp butter to a large non-stick pan over medium heat, then add onions. Season with a little salt and cook for 3-5 minutes.
Add another tbsp of butter to the pan, then add the chard. Season with a pinch of salt and cook for 5-7 minutes or until chard is slightly wilted.
Mix in the lentils, then add the broth and cumin and cook on low or medium-low heat for about 10 minutes so the flavors can develop.
While the lentils cook, pat the fish dry and drizzle about a ½ tbsp of olive oil onto each piece, making sure to get both sides of each filet.
Distribute taco seasoning onto the fish, making sure to season each side of the fish filets. Pat the seasoning into the fish.
Heat 2 tbsp of oil in a cast iron pan (you could also use a non-stick or stainless steel). When the oil is hot, add the fish, cooking about 3-5 minutes on the first side (if your fish is very thick, you may need more time; if it's very thin, you'll need less time).
Flip your fish over to cook the other side, then add 2 tbsp of butter to the pan. Use a spoon to baste the fish with the butter. Cook for about 3-4 minutes on this side, or until the fish registers 130°F to 135°F with an instant read thermometer. Remove from pan and let rest. The fish will continue to cook from residual heat as it rests.
To finish the lentils, add ½ cup water, another tbsp of butter, and the lime juice to the lentil pan. Taste for salt and cumin (add more, if desired).
Remove the lentils from the heat and gently mix in the avocados.
Spoon some lentils and avocado onto a plate and top with a piece of the pan fried cod. Garnish with chives, if desired.