On a 6-quart Instant Pot, select Normal/Medium Sauté. Add the butter and melt. Add the onions, garlic and fennel seeds,
then cook, stirring occasionally, until the onions are lightly browned, 3 to 4 minutes.
Add the wine, ½ cup water, 2 teaspoons salt and ½ teaspoon pepper, then bring to a simmer. Add the lamb and stir to
combine, then distribute in an even layer.
Press Cancel, lock the lid in place and move the pressure valve to Sealing. Select Pressure Cook or Manual; make sure the
pressure level is set to High. Set the cooking time for 25 minutes.
When pressure-cooking is complete, let the pressure reduce naturally for 15 minutes, then quick-release the remaining
steam by moving the pressure valve to Venting. Press Cancel, then carefully open the pot.
Using a large spoon, skim off and discard the fat from the surface. Select Normal/Medium Sauté and bring to a simmer.
Don't bother defrosting the peas before use. They'll thaw and cook through quickly after they're added to the pot. Don't stir the tarragon directly into the stew. The heat causes it to discolor and it loses its freshness. Add at the table for the brightest color and flavor.
Add the green beans and tomatoes, then cook, stirring occasionally, until the beans are tender, 7 to 10 minutes.
Press Cancel to turn off the pot. Stir in the peas and lemon juice, then taste and season with salt and pepper. Serve
sprinkled with the tarragon.