Easy Nigiri Platter (Salmon, Tuna, and Spicy Shrimp Nigiri)

0 Make the Sushi Rice: Rinse the rice until the water runs clear. Cook in a rice cooker according to instructions with the kombu and salt. Stir the sugar and rice vinegar together in a small bowl until dissolved, then fold into the cooked rice.
1 Press and Chill: Line a quarter sheet pan with parchment paper. Spoon in the rice and press it into a firm, even layer. Cover with plastic wrap, weigh it down (a bag of rice works well), and freeze for 20 to 30 minutes until firm.
2 Make the Eel Sauce: Add the soy sauce, sugar, and rice vinegar to a saucepan and simmer until the sauce coats the back of a spoon, 5 to 7 minutes after it begins simmering. Cool in the fridge or an ice bath.
3 Make the Spicy Mayo: Mix the japanese mayo, sriracha, sesame oil, soy sauce, sugar, and chives in a large bowl. Set aside 2 tablespoons in a ramekin for serving. Reserve the rest for the shrimp.
4 Make the Scallion Soy Sauce: Mix the soy sauce, lemon juice, rice vinegar, agave, and scallions in a ramekin.
5 Prepare the Seafood: Bring a pot of water to a boil. Cook the shrimp for 2 to 3 minutes, until bright pink. Mince very small and fold into the bowl of spicy mayo. Chill the salmon and tuna in the freezer for a few minutes, then slice each into 9 thin pieces.
6 Unmold the Rice: Remove the weight and plastic wrap. Lay a sheet of parchment on a cutting board, invert the pan onto it, and lift the pan away. Peel off the pan parchment and transfer the rice slab to a serving platter.
7 Top and Cut: Spread the spicy shrimp evenly over one third of the rice. Arrange the salmon slices in rows over the second third and the tuna over the last third. With an oiled knife, cut into bite-size pieces following the rows, about 6 spicy shrimp bites and 9 each of salmon and tuna, or cut to the size you prefer.
8 Garnish and Serve: Top the spicy shrimp bites with fresno slices, the salmon with chives, and the tuna with jalapeño, avocado, and a drizzle of eel sauce. Serve immediately with the spicy mayo, eel sauce, scallion soy sauce, sesame seeds, and nori.