For polenta, in medium saucepan, bring water to a boil, then add stock cube and butter. Whisk in cornmeal and cook for 15 to 20 minutes, covered, stirring every few minutes, depending on how hot the fire is. If polenta starts to stick or burn, reduce heat or add water. Whisk in Parmesan. Season, to taste, and set aside pot.
Meanwhile, for gremolata, in medium bowl, combine lemon juice and zest with garlic, chopped dill, parsley, and pinch of salt.
Over medium-high, heat 12 inch cast iron skillet. Be patient; the pan is hot enough when you place a hand above it and feel a fair bit of heat, about 3 to 4 minutes. Add tomatoes, still on the vine, and cook until lightly charred but not bursting. Remove tomatoes from pan.
For salmon, add oil to pan. When hot, add salmon, skin side down. Cook until the line of opacity reaches halfway up the side of salmon, about 2 minutes. Flip and cook for 30 seconds to 1 minute longer. If the two lines of opacity meet, the fish will be overcooked, so remove them sooner rather than later.
To serve, plate polenta and fish and top with gremolata, and place tomatoes on the vine alongside. Sprinkle with salt.
Enjoy!