Melt 3 tablespoons of the butter in a large skillet over medium heat. Add the sweet onion, scallions, and both red and yellow bell pepper. Sauté until the onion is translucent and all the vegetables are tender, about 7 to 10 minutes.
Add the garlic and continue cooking for an additional 3 minutes.
Toss in the remaining tablespoon of butter. When the butter has melted add the 3 tablespoons of flour, stirring into the vegetables and cook for about 3 to 5 minutes.
Reduce the heat to medium-low then stir in the cream and the mustard. Toss in the chopped parsley, basil, garlic powder, thyme, kosher salt, and pepper.
Continue cooking, stirring frequently until the sauce has thickened – about 15 minutes. Remove the sauce from the heat and allow it to cool to room temperature.
When the sauce has cooled, pour it into a large mixing bowl. Add the breadcrumbs and mix together.
Gently fold the crab into the sauce and breadcrumbs. The mixture should hold together but if it feels too wet just add more breadcrumbs, about 2 tablespoons at a time.
Using a large ice-cream scoop or spoon, gather up small portions of the crab cake mixture, each about 3-ounces. Place the rounds onto rimmed baking sheets lined with parchment or wax paper, patting each into a small cake about ½” thick. Cover with plastic wrap and chill for at least 3 hours or overnight. You can also pop the pan with the crab cakes into the freezer and flash freeze. Once the crab cakes are frozen, place parchment paper between each one and store in a freezer bag or container.
When you’re ready to cook the crab cakes, warm the olive oil in a large skillet over medium heat. If you’re pulling them from the freezer, let them set out for about 15 minutes before cooking to thaw slightly.
If you want the extra flour breading on the outside, place the additional ½ cup flour into a shallow pan. When the oil is hot, dredge the crab cakes in the flour and place them into the hot pan. Cook on each side until golden brown. The crab cakes can also be cooked without the flour breading.