If you have leftover flowering fresh coriander (cilantro) sprigs from the zucchini dish above, don't let them go to waste. Instead, add them to the garnish of borage flowers here.
Fish: Combine coarse salt and water in a large bowl and whisk until salt dissolves. Place halibut in a deep nonreactive container just wide enough to hold fillets in a single layer, and add lemon and dill stems. Pour salted water over fish to completely submerge, cover container with plastic wrap, and refrigerate at least 2 hours and up to 4 hours.
Tahini sauce: Mash garlic and coarse salt into a paste with a mortar and pestle or the flat side of a knife. Transfer garlic to a medium bowl; whisk in tahini. Whisk in lemon juice, then the water, whisking until sauce is the consistency of lightly whipped cream.
Cucumber and borage: Melt 2 tablespoons butter in a medium saute pan over medium heat. Add cucumber and a pinch of coarse salt; reduce heat to medium-low. Cook, stirring occasionally, until cucumber begins to become translucent, about 10 minutes. Transfer cucumber to a bowl with a slotted spoon. Melt remaining tablespoon butter over medium-low heat, add shallot and garlic, and cook, stirring occasionally, until softened, about 5 minutes. Add borage leaves and stalks and a pinch of coarse salt. Increase heat to medium-high; cook until borage browns in places and stalks become tender, about 4 minutes. Add lemon juice, scraping up browned bits with a wooden spoon, and transfer to bowl with cucumber. Season mixture with coarse salt, gently stirring to combine, and cover to keep warm.
To finish: Remove fish from brine; discard brine. Place fillets, skinned side down, in a single layer in a steamer; top with 1 tablespoon oil and sprinkle with flaky salt. Bring 1 inch of water to a simmer in a large, shallow pot. Place steamer in pot and cover. Steam until fillets are just cooked through and opaque throughout, about 4 minutes.
Spread 1 tablespoon tahini sauce across each of 4 plates and top with fish. Divide cucumber and borage among plates, drizzle with oil and any accumulated juices, and garnish with borage flowers.