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1
Heat the oil in a large frying pan over medium heat until shimmering.
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2
Add the celery, carrot, onion, garlic, and thyme and cook, stirring occasionally, until the carrots just begin to soften, about 10 minutes.
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3
Add the tomato paste, stir to coat the vegetables, and cook until its no longer raw-tasting, about 3 minutes.
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4
Add the wine and mustard, stir to combine, and bring to a simmer.
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5
Cook until the wine reduces by roughly half, about 5 minutes.
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6
Remove from heat and set aside.
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7
(At this point the mixture can be completely cooled and refrigerated up to 12 hours until ready to continue.)
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8
Place the flour, measured salt, and measured pepper in a large bowl and whisk to combine; set aside.
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9
Trim the roast of excess fat and sinew and cut it into 1- to 1-1/2-inch cubes.
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10
Place the meat and mushrooms in the flour mixture and toss to coat.
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11
Pour all of the meat, mushrooms, and loose flour mixture into the slow cooker.
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12
Add the vegetable-wine mixture and stir to combine.
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13
Add the beef broth and bay leaf, tucking the bay leaf into the broth.
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14
Cover and cook on high, stirring about every 1 1/2 hours if possible (if you cant stir it that often, its OK), until the beef is tender and can easily be pierced with a fork, about 5 hours.
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15
Add the peas and parsley, stir to combine, and continue to heat until the peas are warm, about 5 minutes.
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16
Taste and season with more salt and pepper as needed.
