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It's a basic cooking tip to prepare all your ingredients before starting, and when using many spices that need to be added at the same time, which is the rule in Indian-style dishes, it's always a good idea to prepare them all in a bowl that you can then sprinkle altogether. I usually do the same with the water, lemon and salt in this recipe, so I can pour them directly.
Garam masala is an Indian "sweet" spice mix available everywhere in the UK, and it must be similarly easy to find in the spice section of US supermarkets. However, if you can't get your hands on it, you can replace it with (quantities for this recipe
1/3t ground cardamom + 1t ground coriander + 2/3t ground cumin + 1/4t black pepper + 1/4t ground cloves + 1/4t ground cinnamon + 1/4t ground nutmeg.
Or use curry powder, but that's a different taste and will make the recipe spicier than it already is!
More recipes in my folder: Quick Food, Not Fast Food