When a recipe calls for onion, you can accomplish this in one of two ways:
Fresh Onion
Cut by hand or dice finely in chopper/food processor.
Dry Onion, Reconstituted
Onion and dried minced onion can be swapped in recipes.
To reconstitute dried minced onion: place dried minced onion in a small dish. Add cold water to the dish: twice the amount of onion that you used. Place dish in refrigerator for 10-15 minutes, or until the liquid is absorbed. Use in place of fresh chopped/minced onion in any recipe.
Conversion
3 Tbsp dried minced onion + 6 Tbsp water will reconstitute to approximately 1/2 cup fresh minced onion.
2 Tbsp dried minced onion + 4 Tbsp water will reconstitute to approximately 1/3 cup fresh minced onion.
1 1/2 Tbsp (4 1/2 tsp) + 3 Tbsp water will reconstitute to approximately 1/4 cup fresh minced onion.
Reverse to substitute fresh, minced onion in a recipe that calls for reconstituted: 3 Tbsp dry onion, reconstituted would require 1/2 cup fresh to substitute, etc.