Halve the peppers lengthwise, remove the stem and discard the seeds. Put in a large microwave-safe bowl and toss with a splash of water and a large pinch of salt. Cover with plastic wrap and cut a slit in the center to allow steam to escape using a paring knife. Microwave at 100 percent power until the peppers are soft and pliable, 5 to 7 minutes. Allow to cool completely.
Heat 1 tablespoon of oil in a large nonstick skillet over medium-high heat and add the sausages, cut-side down. Cook until browned on both sides, about 3 minutes per side. Transfer to a plate.
Lower the heat to medium, add the onion and remaining tablespoon of oil to the skillet and cook, stirring frequently, until soft and just starting to brown, about 5 minutes. Add the sausages back to the skillet and pour in the marinara and 1/4 cup water. Bring to a low simmer and cook until the sausages and sauce are warmed through and the sauce is slightly thickened, about 3 minutes. Remove from heat, stir in the parsley and set aside.
Arrange the pepper halves on a work surface cut-side up. Put a chicken sausage half in a pepper half with some of the sauce and onions and fold up like a taco. Wrap halfway in wax paper. Repeat with the remaining ingredients to make 3 more tacos and serve immediately.
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