Bring a large pot of water to a boil. Season generously with salt.
Heat the olive oil, crushed red pepper flakes, garlic and spring onions in a large saute pan and cook over medium heat, stirring often, until softened, 5 to 6 minutes. Add the white wine and allow it to reduce, about 1 minute. Add the cream, then let come to a simmer and cook for 3 to 4 minutes to thicken.
Meanwhile, add the pasta to the boiling water and cook to al dente, 2 to 3 minutes. Add the Fresno to the sauce and simmer for another minute. Turn off the heat, then stir the crab and lemon juice into the sauce. Strain the pasta directly into the sauce and gently stir to marry the sauce with the pasta. (Save a little pasta water just in case, to thin the sauce if needed.)
Add the butter, dill, parsley, tarragon and salt to taste; the sauce should begin to glaze the pasta. Once thickened, remove the pan from the heat and season with salt and pepper. If the pasta is too thick, add a splash of reserved pasta water. Sprinkle with Parmesan and drizzle with extra-virgin olive oil.
Combine the flour and salt in a bowl and mix. Place the flour in a mound on a clean work surface. Make a well in the center of the flour. Crack the eggs into the well. Using a fork, whisk the eggs and add the oil. Slowly and carefully mix the flour into the eggs with the fork, making sure not to break through the walls of the flour mound. Once the eggs are incorporated into the flour and a dough starts to form, you can use your hands to incorporate the remaining flour and knead it into a dough. Once the dough is formed, knead for about 5 minutes. (You can also make the dough in a stand mixer.)
Wrap the dough in plastic wrap and let it rest in the refrigerator for 45 minutes to an hour.
On a floured work surface, roll the dough with a rolling pin into a rectangle that is about 1/8-inch thick. Lightly flour the top, then gently roll up the dough into a log. Slice into 1/4-inch-thick slices. Unravel and lay out the fettucine on a lightly floured sheet pan. (You can also run the dough through a pasta machine per the manufacturer's instructions.)