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1
Prepare grill for cooking over medium-hot charcoal (moderate heat for gas). (If using a charcoal grill, open vents on bottom of grill, then light charcoal. When Charcoal turns grayish white (about 15 minutes from lighting), hold your hand 5 inches above grill rack to determine heat for charcoal as follows: Hot: When you can hold your hand there for 1 to 2 seconds; Medium-hot: 3 to 4 seconds; Low: 5 to 6 seconds. If using a gas grill, preheat burners on high, covered, 10 minutes, then, if necessary, reduce to heat specified in recipe.).
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2
While coals are lighting, heat a dry small heavy skillet (preferably cast-iron) over moderately low heat until hot, then toast coriander and cumin seeds, shaking skillet frequently, until a shade darker, 2 to 3 minutes. Transfer seeds to mortar or grinder and cool completely, about 3 minutes, then finely grind.
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3
Transfer ground-spice mixture to a large bowl, then mash into softened butter along with parsley, red pepper flakes (to taste), and garlic until combined well.
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4
Toss mushrooms with oil, salt, and pepper in a bowl, then grill on lightly oiled grill rack, covered only if using a gas grill, turning over occasionally, until lightly browned and tender, 12 to 14 minutes total. Quarter mushrooms while still hot, then toss with butter mixture and salt and pepper to taste.
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5
Cooks' notes: If you aren't able to grill outdoors, mushrooms can be grilled in a hot lightly oiled well-seasoned ridged grill pan over moderately high heat.If you aren't able to find medium mushrooms, you can use small ones but will need to grill them on a lightly oiled perforated grill sheet or slide them onto soaked wooden skewers to prevent them from falling through the grill rack.
