Place pecans, cilantro, garlic, lime or lemon juice, 1/3 cup olive oil and chili flakes in a food processor. Pulse until finely chopped and well combined. Add a little extra oil if you find the pesto too thick.
Transfer pesto to a serving bowl.
Brush each halibut steak with the 2 Tbsp. olive oil; season with salt and pepper. Oil the bars of the grill. Grill the halibut 3 to 4 minutes per side, or until just cooked through. Set fish on serving plates. Top each steak with a generous dollop of pesto and serve.
Note: A medium-sized bunch of cilantro should yield the sprigs required for the recipe. Keep any leftover pesto refrigerated in a tight-sealing jar. It will keep about a week.
Options: Instead of pecans, try unsalted, roasted cashews in the pesto. Instead of halibut steaks, try the pesto on another type of grilled fish, such as salmon steak, or serve it with grilled chicken breast. Instead of using the pesto as a topping, use it to spread on burger buns you fill with grilled fish or chicken and desired accouterments.