While Shavuot is your big chance to indulge in cheesecake, blintzes, eggplant Parmesan, and lots of other very caloric, yummy, cheesy, dairy menu items, grilled fish on the other hand, served with a vegetarian relish on a bed of arugula is super delicious, will impress your guests, and will leave some room and more calories for dessert. 
I purchased a beautiful piece of Chilean Sea Bass, an indulgent choice to be sure, expensive but oh so tasty. Feel free to use salmon, halibut, cod or any other fish popular where you live. Count on about 8 ounces per adult serving for a dinner sized portion. I grilled the fish on an outdoor barbecue, but again, you could pan sauté the fish or bake it if you prefer.
I find the most successful way to cook fish on an outdoor barbecue is to first preheat the grill for about 15 minutes. Spray fish filets on both sides with a natural oil cooking spray. Place fish on hot grill and do not disturb for about 2 to 4 minutes depending on how thick the fish is. When fish is a little charred on one side, flip fish, with a large metal spatula only once. 
Mango Corn Relish
Serves 4 to 6
3 ears of fresh corn on the cob
1 large mango
1 cup red grape tomatoes
1 cup yellow cherry tomatoes
3 bell peppers (red, orange, yellow) charred, peeled, and seeded
1 large Haas avocado
1 Poblano or jalapeño chili
1 teaspoon salt
1 to 2 fresh limes
Heat a large pan filled with water on high heat. When water comes to a boil peel ears of corn and add them to water. Boil for two or three minutes and remove from boiling water. Wait until you can touch the corn and then, cut kernels off cob with a sharp knife. Set kernels aside. Peel mango and slice large pieces off seed. Dice mango pieces into 1/ 2 inch cubes. Place mango in a large mixing bowl. 
Next slice washed and dried tomatoes into quarters and place in mixing bowl. Dice bell peppers and place in mixing bowl.  Peel avocado and dice avocado into pieces about the same size as mango. Mince chili being careful not to include seeds. Add corn kernels to relish in mixing bowl and then, place minced chili on top.  Sprinkle relish with salt to your taste. Squeeze fresh lime over relish, about two tablespoons to your taste. Toss relish together and serve over grilled fish on a bed of wild arugula.