Combine 25 ml each of olive oil and lemon juice. Toss the chicken breasts in the mixture and season both sides with salt and pepper. Broil the chicken breasts for 5 minutes per side, they should be still slightly pink inside. Set aside to cool, then cut into 4-cm pieces.
Cut dark green leaves from leeks and discard. Wash leeks well and slice into 1-cm sections. Clean and slice the mushrooms.
Heat butter in skillet on medium heat. Add the leeks, sliced mushrooms and garlic and sauté for about 3 minutes or until leeks soften. Stir in flour and cook until pale gold, about 4 minutes, adding more butter if needed. Stir in the chicken, chicken stock, tarragon, lemon juice and rind. Bring to boil. Add the cream, reduce heat and simmer for 5 minutes or until thick and glossy. Season well with salt and pepper.
Stir in the frozen peas and simmer for another 1-2 minutes, just until the peas are defrosted.
Serve with biscuits or dumplings, or top it with a light and flaky crust to transform it into a pot pie.