Place a medium saucepan over medium heat, add 3 tbsp of butter & allow to melt. Add the shallot & garlic & sauté until shallot is soft & translucent (~4 min).
Add the flour & cook, whisking until pale golden brown (~30 seconds).
Stir in tomato paste followed by milk, & whisk until combined. Lower heat to medium-low & let simmer, whisking as sauce thickens (~10 min).
Stir in the sun dried tomatoes. Remove from heat then cover & set aside.
Bring a pot of salted water to a boil. Add fettuccine & cook, stirring occasionally, until al dente (~10 min). Reserve 1 cup of pasta cooking water, drain & return pasta to pot.
In a sauté pan over medium heat, add 2 tbsp of olive oil. Add 2 tbsp of butter, allow to melt. Add shrimp & sauté on both sides until no longer translucent (~1½ min per side). Remove shrimp & set aside.
Return pan to heat, add remaining olive oil. Add remaining butter to melt. Working in batches, add mushrooms to pan & sauté, stirring occasionally, until mushrooms shrink. Cook until mushrooms are golden brown (~5-6 min).
Add sauce & mushroom mixture to cooked pasta. Toss until thoroughly coated. Stir in pasta cooking water ¼ cup at a time, tossing to combine. Add more water if needed. Stir in half of the parsley.
Divide fettuccine with sun dried tomato cream sauce among pasta bowls & top with shrimp. Garnish with parsley & serve.