You need more than just butter, but my favorite is still pretty simple.
2 lbs round or chuck, cubed, marinate in vinaigrette dressing and cheap red wine for 1-2 hrs in refrigerator.
1/2 medium onion, diced (whole one if you really like onions)
1 green pepper, cut into bite size pieces
4 oz sliced mushrooms
2 tablespoons butter (or maybe 3 if you think your cholesterol might be too low)
garlic salt
Into a big frying pan add butter, venison, onion, pepper, shrooms, and a sprinkle of garlic salt. Simmer over med-hi heat, stir regularly for 10-12 min, add more wine if it starts to dry out.
All that lacks to be fajitas is the tortillas, guacamole and sour cream. Sounds good to me.
My other preference is to grill it over mesquite charcoal.